Take sprockets-rails out of :assets group

[mayorova]

What this PR does / why we need it:

Fixing a regression from #4145

The issue is:

NoMethodError: undefined method `assets' for #<Rails::Application::Configuration:0x000000000048d0>
Did you mean?  asset_host
/opt/system/vendor/bundle/ruby/3.1.0/gems/railties-7.1.5.2/lib/rails/railtie/configuration.rb:109:in `method_missing'
/opt/system/config/application.rb:189:in `<class:Application>'
/opt/system/config/application.rb:60:in `<module:System>'
/opt/system/config/application.rb:44:in `<top (required)>'

That's because before that PR, while gem 'sprockets-rails' was inside the :assets group, gem 'sass-rails', now removed, was outside (so, applied globally), and as it had sprockets-rails as its dependency anyway, it was available everywhere, and enabled config.assets for Rails configuration.

We only need dartsass-rails and non-digest-assets for example at the assets precompiling stage. However, sprockets-rails is needed in production too, as it actually implements the logic of serving the assets, so I have moved it accordingly.

Which issue(s) this PR fixes

-none-

Verification steps

Building with openshift/system/Dockerfile should work.

Special notes for your reviewer:

[akostadinov]

Makes sense. I'm just wondering if we se the assets gems isolated somewhere. That could be a problem because then we will not have the sprockets. But probably not. Doesn't make too much sense.

[mayorova]

Makes sense. I'm just wondering if we se the assets gems isolated somewhere. That could be a problem because then we will not have the sprockets. But probably not. Doesn't make too much sense.

Hm... I think the ungrouped gems are instlaled always, regardless of whether we use --with or --without in bundler.