add support for .net 9 #499

Job	Run time
check_version_command_net472	3m 6s
check_version_command (net6.0)	29s
check_licenses (net6.0, App)	48s
check_licenses_net472 (App)	2m 16s
check_version_command (net8.0)	27s
check_licenses (net6.0, Tests)	36s
check_licenses_net472 (Tests)	2m 34s
check_version_command (net9.0)	25s
check_licenses (net6.0, ProjectWithReferenceContainingLicenseExpression)	32s
test (ubuntu-latest, net6.0)	1m 41s
check_licenses_net472 (ProjectWithReferenceContainingLicenseExpression)	2m 9s
check_licenses (net8.0, App)	38s
test (ubuntu-latest, net8.0)	1m 33s
check_licenses (net8.0, Tests)	38s
test_windows (net6.0)	4m 17s
test (ubuntu-latest, net9.0)	1m 32s
check_licenses (net8.0, ProjectWithReferenceContainingLicenseExpression)	25s
test_windows (net8.0)	4m 16s
test (macos-latest, net6.0)	2m 51s
check_licenses (net9.0, App)	29s
test_windows (net472)	5m 10s
test (macos-latest, net8.0)	1m 42s
check_licenses (net9.0, Tests)	38s
test_windows (net9.0)	7m 15s
test (macos-latest, net9.0)	1m 49s
check_licenses (net9.0, ProjectWithReferenceContainingLicenseExpression)	26s
	48m 42s

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