MudDialog: Don't override Class param and convert BackgroundClass to getter

[henon]

Description

This PR removes ConfigureInstance() from MudDialogInstance to complete my refactoring towards only using getters instead of caching state in private and protected fields as a followup to #9906 where some fields were already eliminated.

The protected property BackgroundClass is now only used as a getter instead of using it as storage. I am not sure if this is even a breaking change as it is quite unlikely that someone would have derived from MudDialogInstance. But we'll not take any risks and merge it only with v8.

How Has This Been Tested?

No additional tests needed.

Type of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or improvement to the website or code docs)

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[henon]

Overwriting Class is not necessary for the dialog's functionality. Class is already set to _dialog.Class. The razor only uses Classname (consistently to other components) which contains _dialog.Class.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.80%. Comparing base (28bc599) to head (0009a63).
Report is 568 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #9909      +/-   ##
==========================================
+ Coverage   89.82%   90.80%   +0.97%     
==========================================
  Files         412      411       -1     
  Lines       11878    12844     +966     
  Branches     2364     2483     +119     
==========================================
+ Hits        10670    11663     +993     
+ Misses        681      618      -63     
- Partials      527      563      +36     

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