Bump version to "v21.0.0-rc.6" with changelog. #10066

Job	Run time
lint	2m 42s
build	7m 54s
build-e2e-windows-subset	8m 5s
analyze	6s
test	3m 35s
e2e (ubuntu-latest, 22, esbuild, 0)	7m 48s
e2e (ubuntu-latest, 22, npm, 4)	8m 50s
e2e (ubuntu-latest, 22, npm, 1)	11m 7s
e2e (ubuntu-latest, 22, npm, 2)	11m 25s
e2e (ubuntu-latest, 22, npm, 0)	11m 7s
e2e (ubuntu-latest, 22, esbuild, 1)	9m 27s
e2e (ubuntu-latest, 22, esbuild, 2)	9m 39s
e2e (ubuntu-latest, 22, esbuild, 3)	11m 14s
e2e (ubuntu-latest, 22, esbuild, 5)	7m 17s
e2e-package-managers (ubuntu-latest-4core, 22, yarn, 0)	9m 6s
e2e (ubuntu-latest, 22, npm, 5)	10m 16s
e2e-package-managers (ubuntu-latest-4core, 22, pnpm, 0)	7m 4s
e2e (ubuntu-latest, 22, npm, 3)	8m 56s
e2e-package-managers (ubuntu-latest-4core, 22, yarn, 2)	7m 53s
e2e-package-managers (ubuntu-latest-4core, 22, pnpm, 2)	5m 56s
e2e-package-managers (ubuntu-latest-4core, 22, yarn, 1)	6m 35s
e2e-package-managers (ubuntu-latest-4core, 22, pnpm, 1)	5m 15s
e2e (ubuntu-latest, 22, esbuild, 4)	7m 22s
e2e-snapshots	0s
e2e-windows-subset	3m 58s
	3h 2m 37s

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