Update to .NET 9

[martincostello]

• Update to .NET 9.
• Suppress noisy new CA1515 warning.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.71%. Comparing base (4d42365) to head (9f7cc20).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1358      +/-   ##
==========================================
+ Coverage   78.65%   78.71%   +0.05%     
==========================================
  Files         140      140              
  Lines        3556     3556              
  Branches      501      501              
==========================================
+ Hits         2797     2799       +2     
+ Misses        511      510       -1     
+ Partials      248      247       -1     

Flag	Coverage Δ
linux	78.71% <ø> (+0.28%)	:arrow_up:
macos	46.45% <ø> (ø)
windows	46.45% <ø> (ø)

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