BreadcrumbLevel Critical renamed to Fatal

[jamescrosswell]

Resolves #4028:

• Change BreadcrumbLevel.Critical to BreadcrumbLevel.Fatal for consistency with other SDKs #4028

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (version6@33615f4). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/Sentry/Breadcrumb.cs	33.33%	2 Missing ⚠️
src/Sentry.Log4Net/LevelMapping.cs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             version6    #4605   +/-   ##
===========================================
  Coverage            ?   73.08%           
===========================================
  Files               ?      479           
  Lines               ?    17387           
  Branches            ?     3432           
===========================================
  Hits                ?    12707           
  Misses              ?     3820           
  Partials            ?      860           

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[Flash0ver]

@sentry review