agent-platform release v4.156.0

Added

• (components) Select kagent 1.21 (tls:// Agent egress, no loopback token) in #1160 by @teemow

Changed

• (deps) Update agentgateway line to v2.6.5 in #1155 by @renovate[bot]

Full Changelog: https://github.com/giantswarm/agent-platform/compare/v4.155.1...v4.156.0