Update dpage/pgadmin4 Docker tag to v6.21

This MR contains the following updates:

Package	Type	Update	Change
dpage/pgadmin4	ironbank-docker	minor	6.20 -> 6.21
dpage/pgadmin4		minor	6.20 -> 6.21
dpage/pgadmin4	stage	minor	6.20 -> 6.21

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