ci: Bump Emacs 29.x to 3 #18

Job	Run time
build (ubuntu-latest, 26.3, false)	29s
build (ubuntu-latest, 27.2, false)	46s
build (ubuntu-latest, 28.2, false)	51s
build (ubuntu-latest, 29.3, false)	51s
build (macos-latest, 26.3, false)	2m 7s
build (macos-latest, 27.2, false)	2m 22s
build (macos-latest, 28.2, false)	2m 7s
build (macos-latest, 29.3, false)	2m 11s
build (windows-latest, 26.3, false)	2m 46s
build (windows-latest, 27.2, false)	1m 20s
build (windows-latest, 28.2, false)	1m 13s
build (windows-latest, 29.3, false)	1m 4s
build (ubuntu-latest, snapshot, true)	38s
build (macos-latest, snapshot, true)	2m 37s
build (windows-latest, snapshot, true)	33s
	21m 55s

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