fix: Drop support for Emacs 26.x (#27) #27

Job	Run time
test (ubuntu-latest, 27.2, false)	56s
test (ubuntu-latest, 28.2, false)	1m 15s
test (ubuntu-latest, 29.1, false)	1m 6s
test (macos-latest, 27.2, false)	3m 15s
test (macos-latest, 28.2, false)	3m 4s
test (macos-latest, 29.1, false)	2m 59s
test (windows-latest, 27.2, false)	1m 20s
test (windows-latest, 28.2, false)	1m 28s
test (windows-latest, 29.1, false)	1m 4s
test (ubuntu-latest, snapshot, true)	48s
test (macos-latest, snapshot, true)	2m 44s
test (windows-latest, snapshot, true)	36s
	20m 35s

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