Bump minimum supported version to Emacs-28 #645

Job	Run time
pkryger-taps (macos-latest, 29.4)	4m 23s
pkryger-taps (macos-latest, snapshot)	3m 32s
ci-tests (ubuntu-latest, 28.1)	2m 12s
ci-tests (ubuntu-latest, 28.2)	2m 17s
ci-tests (ubuntu-latest, 29.1)	2m 0s
ci-tests (ubuntu-latest, 29.2)	2m 48s
ci-tests (ubuntu-latest, 29.3)	2m 33s
ci-tests (ubuntu-latest, 29.4)	4m 38s
ci-tests (ubuntu-latest, snapshot)	3m 51s
ci-tests (macos-latest, 28.1)	3m 2s
ci-tests (macos-latest, 28.2)	3m 9s
ci-tests (macos-latest, 29.1)	2m 54s
ci-tests (macos-latest, 29.2)	3m 55s
ci-tests (macos-latest, 29.3)	3m 29s
ci-tests (macos-latest, 29.4)	5m 41s
ci-tests (macos-latest, snapshot)	4m 40s
	55m 4s

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