[SqlClient.Tests] Avoid setting OTEL_SEMCONV_STABILITY_OPT_IN

[Kielek]

Follow op: #3592

Changes

[SqlClient.Tests] Avoid setting OTEL_SEMCONV_STABILITY_OPT_IN

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 71.72%. Comparing base (52bd736) to head (05ba731).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3682   +/-   ##
=======================================
  Coverage   71.72%   71.72%           
=======================================
  Files         445      445           
  Lines       17649    17649           
=======================================
  Hits        12658    12658           
  Misses       4991     4991           

Flag	Coverage Δ
unittests-Instrumentation.SqlClient	87.03% <ø> (ø)

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