agent-platform release v4.21.0

Added

• (substrate) The Substrate line moves to v0.0.30-gs.2 — the worker pool can be spread over nodes and zones (giantswarm/agent-platform#472) in #475 by @teemow

Full Changelog: https://github.com/giantswarm/agent-platform/compare/v4.20.1...v4.21.0