Bump version for v0.21.0 release #73

Job	Run time
Run test and build (ubuntu-24.04-arm, aarch64-unknown-linux-gnu)	55s
Run test and build (macos-15-intel, x86_64-apple-darwin)	1m 21s
Run test and build (windows-latest, x86_64-pc-windows-msvc)	2m 9s
Run test and build (macos-15, aarch64-apple-darwin)	1m 7s
Run test and build (macos-14, aarch64-apple-darwin)	1m 3s
Run test and build (ubuntu-latest, x86_64-unknown-linux-gnu)	1m 4s
	7m 39s

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