Update to .NET 9

[martincostello]

• Update to .NET 9.
• Fix/suppress new code analysis warnings.

[codecov[bot]]

Codecov Report

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

Project coverage is 98.52%. Comparing base (6cae7de) to head (fa10607).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
+ Coverage   97.78%   98.52%   +0.73%     
==========================================
  Files          15       15              
  Lines         271      271              
  Branches       37       37              
==========================================
+ Hits          265      267       +2     
+ Misses          3        2       -1     
+ Partials        3        2       -1     

Flag	Coverage Δ
linux	97.78% <ø> (+0.73%)	:arrow_up:
macos	97.78% <ø> (ø)
windows	97.04% <ø> (ø)

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[martincostello]

@costellobot rebase

[martincostello]

@costellobot rebase