Flutter+Firebaseで"Error output from CocoaPods:"が出力された時

# エラー内容

Flutter+FirebaseでGoogle認証を試そうと思い、`pubspec.yaml`に以下を記述して実行したところ`Error output from CocoaPods`が出力されました。

``` pubspec.yaml
dependencies:
  flutter:
    sdk: flutter
  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^1.0.1

  firebase_core: ^0.7.0
  firebase_auth: ^0.20.1
  google_sign_in: ^4.5.9
```

```
Launching lib/main.dart on iPhone 12 Pro Max in debug mode...
Running pod install...
CocoaPods' output:
↳
      Preparing

    Analyzing dependencies

    Inspecting targets to integrate
      Using `ARCHS` setting to build architectures of target `Pods-Runner`: (``)

    Fetching external sources
    -> Fetching podspec for `Flutter` from `Flutter`
    -> Fetching podspec for `firebase_auth` from `.symlinks/plugins/firebase_auth/ios`
    firebase_auth: Using Firebase SDK version '7.3.0' defined in 'firebase_core'
    -> Fetching podspec for `firebase_core` from `.symlinks/plugins/firebase_core/ios`
    firebase_core: Using Firebase SDK version '7.3.0' defined in 'firebase_core'
    -> Fetching podspec for `google_sign_in` from `.symlinks/plugins/google_sign_in/ios`
    -> Fetching podspec for `integration_test` from `.symlinks/plugins/integration_test/ios`

    Resolving dependencies of `Podfile`
      CDN: trunk Relative path: CocoaPods-version.yml exists! Returning local because checking is only perfomed in repo update
    [!] CocoaPods could not find compatible versions for pod "firebase_auth":
      In Podfile:
        firebase_auth (from `.symlinks/plugins/firebase_auth/ios`)

    Specs satisfying the `firebase_auth (from `.symlinks/plugins/firebase_auth/ios`)` dependency were found, but they required a higher minimum deployment target.

    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.6.6/lib/molinillo/resolution.rb:328:in `raise_error_unless_state'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.6.6/lib/molinillo/resolution.rb:310:in `block in unwind_for_conflict'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.6.6/lib/molinillo/resolution.rb:308:in `tap'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.6.6/lib/molinillo/resolution.rb:308:in `unwind_for_conflict'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.6.6/lib/molinillo/resolution.rb:684:in `attempt_to_activate'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.6.6/lib/molinillo/resolution.rb:254:in `process_topmost_state'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.6.6/lib/molinillo/resolution.rb:182:in `resolve'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.6.6/lib/molinillo/resolver.rb:43:in `resolve'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/resolver.rb:94:in `resolve'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/analyzer.rb:1074:in `block in resolve_dependencies'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/user_interface.rb:64:in `section'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/analyzer.rb:1072:in `resolve_dependencies'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/installer/analyzer.rb:124:in `analyze'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:414:in `analyze'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:239:in `block in resolve_dependencies'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/user_interface.rb:64:in `section'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:238:in `resolve_dependencies'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/installer.rb:160:in `install!'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/command/install.rb:52:in `run'
    /Library/Ruby/Gems/2.6.0/gems/claide-1.0.3/lib/claide/command.rb:334:in `run'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/lib/cocoapods/command.rb:52:in `run'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.1/bin/pod:55:in `<top (required)>'
    /usr/local/bin/pod:23:in `load'
    /usr/local/bin/pod:23:in `<main>'

Error output from CocoaPods:
↳

    [!] Automatically assigning platform `iOS` with version `9.0` on target `Runner` because no platform was specified. Please specify a platform for this target in your Podfile. See `https://guides.cocoapods.org/syntax/podfile.html#platform`.

Error running pod install
Error launching application on iPhone 12 Pro Max.
```


# 環境

* PC：M1 Mac
* OS：macOS Big Sur バージョン11.2.1
* Android studio：4.0.1
* ios simulator：iPhone 12 Pro Max

# 解決方法
エラーの中で以下の部分に着目してみます。

```
    Specs satisfying the `firebase_auth (from `.symlinks/plugins/firebase_auth/ios`)` dependency were found, but they required a higher minimum deployment target.
```

```
Error output from CocoaPods:
↳

    [!] Automatically assigning platform `iOS` with version `9.0` on target `Runner` because no platform was specified. Please specify a platform for this target in your Podfile. See `https://guides.cocoapods.org/syntax/podfile.html#platform`.

```

どうやら`firebase_auth`はより新しいデプロイメントターゲットを要求していて、現在は何も指定していないためiOSのバージョンは9.0になっている、ということが読み取れます。

PodfileにてターゲットのPlatformを指定してください、とあるのでPodfileをios->Podfileを開くとPlatformを指定する部分がコメントアウトされています。ここに10.0を指定します。

``` Podfile
# Uncomment this line to define a global platform for your project
# platform :ios, '9.0'
platform :ios, '10.0'
```

保存して再びRunすると正常に起動したことが確認できました。