MudDataGrid: Rename _classname to Classname etc

[ScarletKuro]

Description

Never used conventions to name as _tableClassName, _style etc. Datagrid shouldn't use that.

Since some were in public API surface (protected) it's a breaking change.

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.

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.23%. Comparing base (91e1e59) to head (30b9ed4).
Report is 11 commits behind head on dev.

Files with missing lines	Patch %	Lines
...MudBlazor/Components/DataGrid/MudDataGrid.razor.cs	94.11%	1 Missing and 1 partial ⚠️
src/MudBlazor/Components/DataGrid/Cell.cs	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10149      +/-   ##
==========================================
+ Coverage   91.18%   91.23%   +0.05%     
==========================================
  Files         411      411              
  Lines       12483    12479       -4     
  Branches     2432     2432              
==========================================
+ Hits        11382    11385       +3     
+ Misses        557      552       -5     
+ Partials      544      542       -2     

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[ScarletKuro]

Added to v8.0.0 Migration Guide #9953