[repo/Hangfire] Prepare to .NET9

[Kielek]

Changes

Follow up #2251

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 93.58%. Comparing base (71655ce) to head (b7eec6b).
Report is 583 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2296       +/-   ##
===========================================
+ Coverage   73.91%   93.58%   +19.67%     
===========================================
  Files         267        4      -263     
  Lines        9615       78     -9537     
===========================================
- Hits         7107       73     -7034     
+ Misses       2508        5     -2503     

Flag	Coverage Δ
unittests-Instrumentation.Hangfire	93.58% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...ation/HangfireInstrumentationJobFilterAttribute.cs	90.74% <100.00%> (+4.07%)	⬆️

... and 263 files with indirect coverage changes