[Resources.AWS] Use System.Text.Json source generator with .NET Standard 2.0

[joegoldman2]

Changes

.NET Standard 2.0 target has been added to this package recently, but preprocessor directives have not been updated to use STJ source generator for this target as well.

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 (828f900).
Report is 515 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2185      +/-   ##
==========================================
+ 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/AWSEBSDetector.cs	92.59% <100.00%> (ø)
src/OpenTelemetry.Resources.AWS/AWSEC2Detector.cs	74.28% <100.00%> (ø)
...enTelemetry.Resources.AWS/ResourceDetectorUtils.cs	100.00% <100.00%> (ø)

... and 273 files with indirect coverage changes