Update rustfmt's default edition: 2018 -> 2021 #274

Job	Run time
test (ubuntu-latest, 28.2)	57s
test (ubuntu-latest, 29.1)	43s
test (ubuntu-latest, 29.2)	56s
test (ubuntu-latest, 29.3)	52s
test (macos-latest, 28.2)	2m 30s
test (macos-latest, 29.1)	2m 10s
test (macos-latest, 29.2)	2m 19s
test (macos-latest, 29.3)	2m 37s
test (windows-latest, 28.2)	1m 51s
test (windows-latest, 29.1)	2m 8s
test (windows-latest, 29.2)	3m 9s
test (windows-latest, 29.3)	2m 1s
test (ubuntu-latest, snapshot, true)	39s
test (macos-latest, snapshot, true)	2m 9s
	25m 1s

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