fix: Drop support for Emacs 26.x #482

Job	Run time
test (ubuntu-latest, 27.1, false)	1m 4s
test (ubuntu-latest, 27.2, false)	1m 23s
test (ubuntu-latest, 28.1, false)	1m 3s
test (ubuntu-latest, 29.1, false)	59s
test (macos-latest, 27.1, false)	3m 2s
test (macos-latest, 27.2, false)	3m 46s
test (macos-latest, 28.1, false)	4m 57s
test (macos-latest, 29.1, false)	2m 57s
test (windows-latest, 27.1, false)	1m 40s
test (windows-latest, 27.2, false)	4m 9s
test (windows-latest, 28.1, false)	1m 30s
test (windows-latest, 29.1, false)	1m 58s
test (ubuntu-latest, snapshot, true)	54s
test (macos-latest, snapshot, true)	2m 48s
test (windows-latest, snapshot, true)	1m 4s
	33m 14s

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