Upgrade rails_admin gem to version 3

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### Problem

`rails_admin` gem is blocking our upgrade to Rails 7:

```shell
Bundler could not find compatible versions for gem "rails":
  In Gemfile:
    rails (~> 7.0.4)

    rails_admin (~> 2.2.0) was resolved to 2.2.1, which depends on
      rails (< 7, >= 5.0)
```

In order to upgrade CustomersDot to Rails 7 we have to upgrade `rails_admin` gem to version 3. We use version `2.2.0` but version `3.1.0` is available and supports Rails 7 - https://github.com/railsadminteam/rails_admin/blob/master/rails_admin.gemspec#L13

Unfortunately there is not straightforwad upgrade path for this gem as new version has [many breaking changes](https://github.com/railsadminteam/rails_admin/blob/master/CHANGELOG.md#300beta---2021-12-20). Attempting to perform an upgrade in a dumb way:

```shell
### Upgrading RailsAdmin from 2.x.x to 3.x.x ###

    Due to introduction of Webpack/Webpacker support, some additional dependencies and configuration will be needed.
    Running `bin/rails g rails_admin:install` will suggest required changes, based on the current setup of your app.
```

Results in a missing method `assets`:

```shell
/Users/vslobodin/.asdf/installs/ruby/2.7.7/lib/ruby/gems/2.7.0/gems/railties-6.1.7/lib/rails/railtie/configuration.rb:97:in `method_missing': undefined method `assets' for #<Rails::Application::Configuration:0x0000000128e96ca8>
Did you mean?  asset_host (NoMethodError)
```

Which indicates we have to load `sprockets`. Applying a small patch for that:

```diff
diff --git a/config/application.rb b/config/application.rb
index cb0b3a821..1a4fc7e26 100644
--- a/config/application.rb
+++ b/config/application.rb
@@ -13,6 +13,7 @@
 require 'active_job/railtie'
 require 'active_storage/engine'
 require 'prometheus/client'
+require 'sprockets/railtie'
 
 # Require the gems listed in Gemfile, including any gems
 # you've limited to :test, :development, or :production.
```

Results in a missing `sassc`:

```shell
cannot load such file -- sassc
```

Adding it (`gem 'sassc-rails'`) makes the application starting at least but attempting to visit any admin page result in:

```
Showing /Users/vslobodin/.asdf/installs/ruby/2.7.7/lib/ruby/gems/2.7.0/gems/rails_admin-3.1.0/app/views/rails_admin/main/_dashboard_history.html.erb where line #22 raised:

uninitialized constant RailsAdmin::Adapters::ActiveRecord::AbstractObject
Did you mean?  RailsAdmin::AbstractModel
```

This happens because `AbstractObject` was deprecated in favour of plain ActiveRecord https://github.com/railsadminteam/rails_admin/commit/af88091d11590dc95df4866c62fba0c49a7bb9a8

To sum up, to achieve this upgrade we need to:

- apply all patches above or find another way to workaround mentioned issues.
- validate we load all JS and CSS correctly
- validate the whole RailsAdmin works with new version
- check and fix our tests in `spec/feature/rails_admin`

### Availability & Testing

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What risks does this change pose to our availability? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing?

Please list the test areas (unit, integration and end-to-end) that needs to be added or updated to ensure that this feature will work as intended. Please use the list below as guidance.
* Unit test changes
* Integration test changes
* End-to-end test change

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It is imperative to monitor Staging end-to-end test runs once this gets merged.

Story points: 4.0