[Resources.OperatingSystem] Replace .NET 6 target with .NET 8 and add .NET Standard 2.0 target

[joegoldman2]

Changes

Replace .NET 6 target, which will be very soon out of support, with .NET 8 and add support for .NET Standard 2.0.

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 77.20%. Comparing base (71655ce) to head (0e340f3).
Report is 501 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2169      +/-   ##
==========================================
+ Coverage   73.91%   77.20%   +3.29%     
==========================================
  Files         267        3     -264     
  Lines        9615      136    -9479     
==========================================
- Hits         7107      105    -7002     
+ Misses       2508       31    -2477     

Flag	Coverage Δ
unittests-Resources.OperatingSystem	77.20% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...sources.OperatingSystem/OperatingSystemDetector.cs	82.25% <100.00%> (ø)

... and 266 files with indirect coverage changes