[Resources.Gcp] Replace .NET 6 target with .NET 8

[joegoldman2]

Changes

Replace .NET 6 target, which will be very soon out of support, with .NET 8.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, 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

All modified and coverable lines are covered by tests ✅

Project coverage is 72.54%. Comparing base (71655ce) to head (4ea031c).
Report is 503 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2167      +/-   ##
==========================================
- Coverage   73.91%   72.54%   -1.37%     
==========================================
  Files         267        2     -265     
  Lines        9615       51    -9564     
==========================================
- Hits         7107       37    -7070     
+ Misses       2508       14    -2494     

Flag	Coverage Δ
unittests-Resources.Gcp	72.54% <ø> (?)

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

see 266 files with indirect coverage changes