Bump version to 21.2.1 #4140

Job	Run time
cackle	1m 11s
check-git-rev-deps	8s
cargo-deny (advisories)	38s
build-and-test (msrv, 20, ubuntu-latest, wasm32-unknown-unknown, false)	2m 49s
build-and-test (msrv, 20, ubuntu-latest, x86_64-unknown-linux-gnu, true)	2m 47s
cargo-deny (bans licenses sources)	38s
fmt	8s
build-and-test (msrv, 21, ubuntu-latest, wasm32-unknown-unknown, false)	2m 48s
build-and-test (msrv, 21, ubuntu-latest, x86_64-unknown-linux-gnu, true)	2m 51s
semver-checks	3m 52s
build-and-test (latest, 20, ubuntu-latest, wasm32-unknown-unknown, false)	2m 36s
build-and-test (latest, 20, ubuntu-latest, x86_64-unknown-linux-gnu, true)	2m 33s
build-and-test (latest, 21, ubuntu-latest, wasm32-unknown-unknown, false)	2m 35s
build-and-test (latest, 21, ubuntu-latest, x86_64-unknown-linux-gnu, true)	2m 52s
publish-dry-run (ubuntu-latest, wasm32-unknown-unknown) / publish-dry-run / publish-dry-run	46m 44s
publish-dry-run (ubuntu-latest, x86_64-unknown-linux-gnu) / publish-dry-run / publish-dry-run	48m 36s
complete	1s
	2h 3m 47s

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