agent-platform release v4.15.5

Changed

• (lines) Re-pin the three upstream lines together — Substrate v0.0.30-gs.1, kagent v0.11.0-gs.14, the agentgateway data plane v1.5.1-gs.4 in #461 by @teemow

Full Changelog: https://github.com/giantswarm/agent-platform/compare/v4.15.4...v4.15.5