[Resources.AWS] Remove .NET 6 target and add .NET Standard 2.0

[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 77.85%. Comparing base (71655ce) to head (78aa4be).
Report is 508 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2164      +/-   ##
==========================================
+ Coverage   73.91%   77.85%   +3.93%     
==========================================
  Files         267       12     -255     
  Lines        9615      289    -9326     
==========================================
- Hits         7107      225    -6882     
+ Misses       2508       64    -2444     

Flag	Coverage Δ
unittests-Resources.AWS	77.85% <100.00%> (?)

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

Files with missing lines	Coverage Δ
src/OpenTelemetry.Resources.AWS/AWSECSDetector.cs	87.87% <ø> (ø)
src/OpenTelemetry.Resources.AWS/AWSEKSDetector.cs	57.89% <100.00%> (ø)
...etry.Resources.AWS/AWSResourceBuilderExtensions.cs	0.00% <ø> (ø)
...enTelemetry.Resources.AWS/ResourceDetectorUtils.cs	100.00% <ø> (ø)

... and 272 files with indirect coverage changes

[CodeBlanch]

LGTM