chore(main): release google-cloud-secret_manager-v1beta1 0.21.0 #105371

Job	Run time
CI (ubuntu-latest, 3.2, --test)	1m 14s
CI (ubuntu-latest, 3.1, --test)	51s
CI (ubuntu-latest, 4.0, --rubocop-toplevel --rubocop --build --yard --linkinator)	1m 18s
CI (ubuntu-latest, 4.0, --test)	58s
CI (ubuntu-latest, 3.4, --test)	1m 8s
CI (macos-latest, 4.0, --test)	1m 0s
CI (windows-latest, 4.0, --test)	2m 54s
CI (ubuntu-latest, 3.3, --test)	1m 5s
	10m 28s

Provide feedback