Bump version to 21.2.1 #4146

Job	Run time
cackle	1m 24s
cargo-deny (advisories)	48s
cargo-deny (bans licenses sources)	43s
fmt	12s
check-git-rev-deps	10s
build-and-test (msrv, 20, ubuntu-latest, wasm32-unknown-unknown, false)	7m 58s
semver-checks	3m 48s
build-and-test (msrv, 20, ubuntu-latest-16-cores, x86_64-unknown-linux-gnu, true)	18m 43s
build-and-test (msrv, 21, ubuntu-latest, wasm32-unknown-unknown, false)	7m 54s
build-and-test (msrv, 21, ubuntu-latest-16-cores, x86_64-unknown-linux-gnu, true)	18m 23s
build-and-test (latest, 20, ubuntu-latest, wasm32-unknown-unknown, false)	7m 32s
build-and-test (latest, 20, ubuntu-latest-16-cores, x86_64-unknown-linux-gnu, true)	17m 7s
build-and-test (latest, 21, ubuntu-latest, wasm32-unknown-unknown, false)	7m 29s
build-and-test (latest, 21, ubuntu-latest-16-cores, x86_64-unknown-linux-gnu, true)	17m 10s
publish-dry-run (ubuntu-latest, wasm32-unknown-unknown) / publish-dry-run / publish-dry-run	47m 22s
publish-dry-run (ubuntu-latest, x86_64-unknown-linux-gnu) / publish-dry-run / publish-dry-run	46m 40s
complete	1s
	3h 23m 24s

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