Update version to v0.9.21. #1381

Job	Run time
Linux GCC Sanitizer Test (ubuntu:noble, address-sanitizer)	24m 24s
Linux GCC Test (debug, debian:bookworm-slim)	6m 0s
Linux GCC Test (debug, ubuntu:noble)	23m 22s
Linux GCC Test (debug, ubuntu:jammy)	22m 29s
Linux GCC Test (release, ubuntu:jammy)	20m 4s
Linux GCC Sanitizer Test (debian:trixie-slim, address-sanitizer)	25m 21s
Linux GCC Sanitizer Test (debian:bookworm-slim, thread-sanitizer)	17m 24s
Linux GCC Test (release, debian:bookworm-slim)	23m 31s
Linux GCC Sanitizer Test (debian:trixie-slim, thread-sanitizer)	16m 56s
Linux GCC Test (release, debian:trixie-slim)	21m 35s
Linux GCC Test (release, ubuntu:noble)	21m 54s
Linux GCC Sanitizer Test (ubuntu:jammy, thread-sanitizer)	3h 0m 42s
Linux GCC Sanitizer Test (debian:bookworm-slim, address-sanitizer)	49m 2s
Linux GCC Sanitizer Test (ubuntu:jammy, address-sanitizer)	24m 28s
Linux GCC Test (debug, debian:trixie-slim)	21m 27s
Linux GCC Sanitizer Test (ubuntu:noble, thread-sanitizer)	17m 1s
Linux Distributable Dispatch	0s
	8h 35m 40s

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