Bump version to "v21.0.0-rc.4" with changelog. #9961

Job	Run time
lint	2m 23s
analyze	7s
build	5m 40s
build-e2e-windows-subset	5m 50s
test	5m 47s
e2e (ubuntu-latest, 22, npm, 0)	11m 9s
e2e (ubuntu-latest, 22, npm, 5)	9m 59s
e2e (ubuntu-latest, 22, npm, 3)	8m 48s
e2e (ubuntu-latest, 22, npm, 1)	11m 42s
e2e (ubuntu-latest, 22, npm, 2)	11m 12s
e2e (ubuntu-latest, 22, esbuild, 0)	9m 16s
e2e (ubuntu-latest, 22, npm, 4)	9m 6s
e2e (ubuntu-latest, 22, esbuild, 4)	6m 36s
e2e (ubuntu-latest, 22, esbuild, 2)	9m 25s
e2e (ubuntu-latest, 22, esbuild, 1)	9m 37s
e2e (ubuntu-latest, 22, esbuild, 3)	9m 14s
e2e-package-managers (ubuntu-latest-4core, 22, yarn, 0)	8m 19s
e2e-package-managers (ubuntu-latest-4core, 22, yarn, 2)	7m 30s
e2e-package-managers (ubuntu-latest-4core, 22, pnpm, 1)	4m 54s
e2e (ubuntu-latest, 22, esbuild, 5)	7m 7s
e2e-package-managers (ubuntu-latest-4core, 22, pnpm, 0)	6m 31s
e2e-package-managers (ubuntu-latest-4core, 22, yarn, 1)	6m 25s
e2e-package-managers (ubuntu-latest-4core, 22, pnpm, 2)	5m 57s
e2e-snapshots	0s
e2e-windows-subset	3m 33s
	2h 56m 7s

Provide feedback