Add net9.0 to AoT test matrix

[martincostello]

Relates to #5518

Changes

Add .NET 9 to the AoT test matrix to try and reproduce #5518.

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 85.52%. Comparing base (6250307) to head (e35d4bb).
Report is 179 commits behind head on main.

❗ Current head e35d4bb differs from pull request most recent head 5af5a3c. Consider uploading reports for the commit 5af5a3c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5519      +/-   ##
==========================================
+ Coverage   83.38%   85.52%   +2.14%     
==========================================
  Files         297      289       -8     
  Lines       12531    12588      +57     
==========================================
+ Hits        10449    10766     +317     
+ Misses       2082     1822     -260     

Flag	Coverage Δ
unittests	?
unittests-Instrumentation-Experimental	24.13% <ø> (?)
unittests-Instrumentation-Stable	24.17% <ø> (?)
unittests-Solution-Experimental	85.31% <ø> (?)
unittests-Solution-Stable	85.49% <ø> (?)

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see 64 files with indirect coverage changes

[github-actions]

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