[Instrumentation.StackExchangeRedis] Fix tests on .NET9

[Kielek]

Fixes #2324

Changes

[Instrumentation.StackExchangeRedis] Fix tests on .NET9

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)

[Kielek]

@xiang17, FYI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.21%. Comparing base (71655ce) to head (1d08c69).
Report is 599 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2327      +/-   ##
==========================================
- Coverage   73.91%   71.21%   -2.70%     
==========================================
  Files         267        5     -262     
  Lines        9615      271    -9344     
==========================================
- Hits         7107      193    -6914     
+ Misses       2508       78    -2430     

Flag	Coverage Δ
unittests-Instrumentation.StackExchangeRedis	71.21% <ø> (?)

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

see 264 files with indirect coverage changes