[Instrumentation.Runtime] Remove legacy checks that only applied for .NET 6

[xiang17]

Fixes #
Design discussion issue #

Changes

Removed checks that was only needed when .NET 6 was supported

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 100.00%. Comparing base (71655ce) to head (a3938cc).
Report is 610 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #2325       +/-   ##
============================================
+ Coverage   73.91%   100.00%   +26.08%     
============================================
  Files         267         2      -265     
  Lines        9615       142     -9473     
============================================
- Hits         7107       142     -6965     
+ Misses       2508         0     -2508     

Flag	Coverage Δ
unittests-Instrumentation.Runtime	100.00% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...elemetry.Instrumentation.Runtime/RuntimeMetrics.cs	100.00% <100.00%> (ø)

... and 263 files with indirect coverage changes

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