CI: Bump actions/upload-artifact from 5 to 6 #1068

Job	Run time
build-and-test (macos-14, 15.4, true, vimtags, check-xcodeproj-compat) / build-and-test	1m 31s
build-and-test (macos-15, 16.4, false, true, true) / build-and-test	1s
build-and-test (macos-13-xcode14-self-hosted, true, _10.9, true, true) / build-and-test	0s
	1m 32s

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