[Exporter.Geneva] Replace hardcoded TFM in test projects

[joegoldman2]

Changes

#2123 introduced a new MSBuild prop SupportedNetTargetsWithoutNet6 used in test projects. This PR uses it in Geneva test projects.

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 58.94%. Comparing base (71655ce) to head (1d57555).
Report is 467 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2128       +/-   ##
===========================================
- Coverage   73.91%   58.94%   -14.98%     
===========================================
  Files         267       44      -223     
  Lines        9615     3924     -5691     
===========================================
- Hits         7107     2313     -4794     
+ Misses       2508     1611      -897     

Flag	Coverage Δ
unittests-Exporter.Geneva	58.94% <ø> (?)

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see 249 files with indirect coverage changes