[repo] Remove .NET 7 target

[xiang17]

Fixes #5639.
Design discussion issue #5639.

Changes

Please provide a brief description of the changes here.

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 86.29%. Comparing base (6250307) to head (d609a57).
Report is 297 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5795      +/-   ##
==========================================
+ Coverage   83.38%   86.29%   +2.90%     
==========================================
  Files         297      256      -41     
  Lines       12531    11138    -1393     
==========================================
- Hits        10449     9611     -838     
+ Misses       2082     1527     -555     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	85.93% <ø> (?)
unittests-Project-Stable	86.18% <ø> (?)
unittests-Solution	86.24% <ø> (?)
unittests-UnstableCoreLibraries-Experimental	85.76% <ø> (?)

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see 207 files with indirect coverage changes