Repository: material-ui
Issue: [Select] Recent removal of `.MuiSelect-root` class prevents specifically targeting root for only `Select`
Labels: waiting for 👍,component: select,status: expected behavior

### Duplicates

- [X] I have searched the existing issues

### Latest version

- [X] I have tested the latest version

### Current behavior 😯

Following the merge of this [PR](https://github.com/mui-org/material-ui/pull/29593) into version 5.1.1, we can no longer specifically target the root element of a Select explicitly without also targeting all Inputs. Having this root class for example allowed you to be able to add different padding for a Select component at the root but not for other Inputs.

I understand it was removed as a result of also removing the `root` override slot because that usage was unclear - however I think we should still keep the class as that is useful for specificity purposes.

### Expected behavior 🤔

`.MuiSelect-root` is still available as a class on the root element but the slot is not available to apply styleOverrides to.

### Steps to reproduce 🕹

Before Select root was removed (v5.1.0): [codesanbox](https://codesandbox.io/s/elated-cache-3ytwj?file=/src/theme.tsx)

After Select root was removed (v5.2.4): [codesandbox](https://codesandbox.io/s/sharp-butterfly-td4el?file=/src/theme.tsx)

If you compare the two you will see that erroneous padding is present on the right side of the select in the "After" option because it is no longer able to target the Select root to override this.

### Context 🔦

Without this root class style overrides get very complicated, very quickly if you are trying to apply varying styles to selects vs inputs vs multiline inputs and there are some styles you may not want just on the input element itself, but rather the wrapper (root). This has also broken our Select styles and prevented us from upgrading beyond 5.1.0 unless we do quite a bit of unconventional style hacking to get our theme back to where it was.

### Your environment 🌎

<details>
  <summary>`npx @mui/envinfo`</summary>

```
  System:
    OS: macOS 11.6
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 169.88 MB / 32.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.17.6 - ~/.nvm/versions/node/v14.17.6/bin/node
    Yarn: 1.22.15 - /usr/local/bin/yarn
    npm: 6.14.15 - ~/.nvm/versions/node/v14.17.6/bin/npm
  Managers:
    Homebrew: 3.3.7 - /usr/local/bin/brew
    pip3: 21.2.4 - /usr/local/bin/pip3
    RubyGems: 3.0.3 - /usr/bin/gem
  Utilities:
    Make: 3.81 - /usr/bin/make
    GCC: 4.2.1 - /usr/bin/gcc
    Git: 2.30.1 - /usr/bin/git
    Clang: 13.0.0 - /usr/bin/clang
  Servers:
    Apache: 2.4.48 - /usr/sbin/apachectl
  Virtualization:
    Docker: 20.10.11 - /usr/local/bin/docker
  IDEs:
    IntelliJ: 2021.2.2
    Nano: 2.0.6 - /usr/bin/nano
    VSCode: 1.62.3 - /usr/local/bin/code
    Vim: 8.2 - /usr/bin/vim
    Xcode: /undefined - /usr/bin/xcodebuild
  Languages:
    Bash: 3.2.57 - /bin/bash
    Java: 1.8.0_302 - /Users/steven.rathbauer/.sdkman/candidates/java/current/bin/javac
    Perl: 5.30.2 - /usr/bin/perl
    PHP: 7.3.29 - /usr/bin/php
    Python: 2.7.16 - /usr/bin/python
    Python3: 3.9.7 - /usr/local/bin/python3
    Ruby: 2.6.3 - /usr/bin/ruby
  Databases:
    SQLite: 3.32.3 - /usr/bin/sqlite3
  Browsers:
    Firefox Developer Edition: 95.0
    Safari: 15.1
  Monorepos:
    Yarn Workspaces: 1.22.15
    Lerna: 4.0.0
```
</details>