Merge remote-tracking branch 'origin/master'

Add NET 9 support, drop NET 6 support (RicoSuter#5026)

Co-authored-by: Rico Suter <[email protected]>

Fix loading of correct JSON serializer options, fixes RicoSuter#4834,…

… regression of RicoSuter#4733, v14.0.7

Remove MemberNotNull and use pragma instead, fixes RicoSuter#4704, fixes

 RicoSuter#4833, v14.0.6

Fix WithOpenApi, v14.0.5, closes RicoSuter#4832