.NET 9 preparation

[martincostello]

• Move suppressions to .ruleset file.
• Set NuGetAuditMode=direct.

Cherry-picked from #1358.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.25%. Comparing base (f8e99bc) to head (deb4356).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1466      +/-   ##
==========================================
+ Coverage   79.22%   79.25%   +0.03%     
==========================================
  Files         138      138              
  Lines        3234     3234              
  Branches      448      448              
==========================================
+ Hits         2562     2563       +1     
+ Misses        441      440       -1     
  Partials      231      231              

Flag	Coverage Δ
linux	79.25% <ø> (+0.03%)	⬆️
macos	47.92% <ø> (ø)
windows	?

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[hwoodiwiss]

The discussion for this CVE indicates that this version is unaffected
GHSA-hh2w-p6rv-4g7w

There's a suggestion to add NoWarn="NU1903" to the package reference to suppress this, which sounds reasonable, at least until the CVE is updated/fixed

[martincostello]

Yeah, I just haven't got around to circling back and fixing this PR yet as I've had a bunch broken this preview.

[martincostello]

I think the GHSA had the wrong versions earlier this week and has since been updated (there was an edit 20 hours ago), as re-running the build with no changes now doesn't raise the warning.