[ASP.NET Core] Remove netstandard2.1 target

[vishweshbankwar]

Fixes #
Design discussion issue #

Changes

Removes netstandard2.1 target.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Merging #5094 (cac7edb) into main (6a829dd) will increase coverage by 0.15%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

❗ Current head cac7edb differs from pull request most recent head 3a306c6. Consider uploading reports for the commit 3a306c6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5094      +/-   ##
==========================================
+ Coverage   83.15%   83.30%   +0.15%     
==========================================
  Files         296      296              
  Lines       12340    12326      -14     
==========================================
+ Hits        10261    10268       +7     
+ Misses       2079     2058      -21     

Flag	Coverage Δ
unittests	83.30% <ø> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...ion.AspNetCore/AspNetCoreInstrumentationOptions.cs	100.00% <ø> (ø)
...tation.AspNetCore/Implementation/HttpInListener.cs	88.88% <ø> (+13.88%)	⬆️

... and 11 files with indirect coverage changes