Bump version to 21.2.0 #448

Job	Run time
build-and-test (msrv)	2m 31s
build-and-test (latest)	2m 18s
fmt	10s
publish-dry-run (macos-latest, aarch64-apple-darwin, --feature-powerset) / publish-dry-run	1m 42s
publish-dry-run (macos-latest, x86_64-apple-darwin, --feature-powerset) / publish-dry-run	1m 50s
publish-dry-run (ubuntu-latest, aarch64-unknown-linux-gnu, --feature-powerset) / publish-dry-run	1m 27s
publish-dry-run (ubuntu-latest, x86_64-unknown-linux-gnu, --feature-powerset) / publish-dry-run	56s
publish-dry-run (windows-latest-8-cores, x86_64-pc-windows-msvc, --features opt --ignore-unknown-... / publish-dry-run	3m 24s
publish-dry-run (windows-latest-8-cores, x86_64-pc-windows-msvc) / publish-dry-run	2m 55s
complete	1s
	17m 14s

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