Warn on older versions of func #21

Job	Run time
test-custom-name	4s
test-default-latest (ubuntu-latest)	4s
test-default-latest (macos-latest)	7s
test-invalid-version	7s
test-default-latest (windows-latest)	16s
test-custom-binary-source	5s
test-custom-version	5s
test-custom-destination	6s
	54s

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