ci: Bump Emacs 29.x to 3 #458

Job	Run time
build (ubuntu-latest, 26.3, false)	32s
build (ubuntu-latest, 27.2, false)	36s
build (ubuntu-latest, 28.2, false)	40s
build (ubuntu-latest, 29.3, false)	46s
build (macos-latest, 26.3, false)	1m 42s
build (macos-latest, 27.2, false)	1m 42s
build (macos-latest, 28.2, false)	2m 30s
build (macos-latest, 29.3, false)	1m 59s
build (windows-latest, 26.3, false)	1m 34s
build (windows-latest, 27.2, false)	2m 0s
build (windows-latest, 28.2, false)	51s
build (windows-latest, 29.3, false)	1m 1s
build (ubuntu-latest, snapshot, true)	38s
build (macos-latest, snapshot, true)	2m 0s
build (windows-latest, snapshot, true)	50s
	19m 21s

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