MudFileUpload: Make FileUploadButtonTemplateContext.ClearAsync required

[ScarletKuro]

Description

This improves #6535
It was already meant to be always set by the FileUploadButtonTemplateContext and the nullable was surpressed with the null! annotation, but it's not required anymore with the required keyword. Now we can use it because of .net6 drop, we could use it before but it would require a special polifyll.

It slightly breaking in terms if someone used this class by himself, but usually this was totally managed by the FileUploadButtonTemplateContext.

Types 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)

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

All modified and coverable lines are covered by tests ✅

Project coverage is 89.47%. Comparing base (6ae6b98) to head (107b1cb).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8547      +/-   ##
==========================================
- Coverage   89.51%   89.47%   -0.05%     
==========================================
  Files         411      411              
  Lines       11845    11845              
  Branches     2351     2351              
==========================================
- Hits        10603    10598       -5     
- Misses        721      724       +3     
- Partials      521      523       +2     

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