[Resources.Azure] 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 83.89%. Comparing base (71655ce) to head (72fed8d).
Report is 496 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2165      +/-   ##
==========================================
+ Coverage   73.91%   83.89%   +9.97%     
==========================================
  Files         267        6     -261     
  Lines        9615      149    -9466     
==========================================
- Hits         7107      125    -6982     
+ Misses       2508       24    -2484     

Flag	Coverage Δ
unittests-Resources.Azure	83.89% <ø> (?)

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see 270 files with indirect coverage changes