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Drop support for .NET 6, test against .NET 9 #990

Job	Run time
cleanup-code (ubuntu-latest)	10m 2s
inspect-code (ubuntu-latest)	6m 11s
build-and-test (ubuntu-latest)	7m 2s
cleanup-code (windows-latest)	9m 46s
inspect-code (windows-latest)	9m 46s
cleanup-code (macos-latest)	10m 2s
build-and-test (windows-latest)	10m 3s
inspect-code (macos-latest)	5m 26s
build-and-test (macos-latest)	6m 25s
publish	1s
	1h 14m 44s

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