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Drop support for .NET 6, test against .NET 9 #992

Job	Run time
inspect-code (ubuntu-latest)	5m 47s
build-and-test (ubuntu-latest)	7m 0s
cleanup-code (ubuntu-latest)	9m 15s
inspect-code (windows-latest)	12m 6s
build-and-test (windows-latest)	13m 7s
cleanup-code (windows-latest)	14m 51s
inspect-code (macos-latest)	4m 17s
build-and-test (macos-latest)	8m 32s
cleanup-code (macos-latest)	7m 22s
publish	3s
	1h 22m 20s

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