Bump version to 21.6.0 #4181

Job	Run time
build-fuzz	3m 19s
check-git-rev-deps	7s
cargo-deny (advisories)	38s
build-and-test (msrv, ubuntu-latest, x86_64-unknown-linux-gnu)	7m 44s
cargo-deny (bans licenses sources)	36s
build-and-test (msrv, ubuntu-latest, aarch64-unknown-linux-gnu, true)	4m 44s
docs	4m 59s
fmt	14s
build-and-test (msrv, windows-latest, x86_64-pc-windows-msvc)	15m 20s
build-and-test (latest, ubuntu-latest, x86_64-unknown-linux-gnu)	7m 8s
readme	5m 33s
semver-checks	5m 19s
build-and-test (latest, ubuntu-latest, aarch64-unknown-linux-gnu, true)	4m 36s
build-and-test (latest, windows-latest, x86_64-pc-windows-msvc)	16m 39s
publish-dry-run (ubuntu-latest, x86_64-unknown-linux-gnu, --feature-powerset --exclude-features d... / publish-dry-run / publish-dry-run	5m 57s
publish-dry-run (ubuntu-latest, wasm32-unknown-unknown) / publish-dry-run / publish-dry-run	4m 50s
complete	1s
	1h 27m 44s

Provide feedback