Raised minimum supported version to Emacs 27.1 #475

Job	Run time
build (ubuntu-latest, 27.2, false)	1m 15s
build (ubuntu-latest, 28.2, false)	44s
build (ubuntu-latest, 29.4, false)	59s
build (macos-latest, 28.2, false)	1m 36s
build (macos-latest, 29.4, false)	2m 20s
build (windows-latest, 27.2, false)	2m 39s
build (windows-latest, 28.2, false)	2m 27s
build (windows-latest, 29.4, false)	2m 38s
build (ubuntu-latest, snapshot, true)	38s
build (macos-latest, snapshot, true)	1m 45s
build (windows-latest, snapshot, true)	2m 2s
	19m 3s

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