Automate release preparation and backports

[DariuszPorowski]

Parent: #11777

Implementation plan: PR 12 — Prepare Release workflow and backport automation

Design: GoReleaser Release Lifecycle

Implementation PR: Not yet created.

Scope

Add a Prepare Release workflow that applies the fixed version policy, renders changelog and release notes, updates versions.yaml and CHANGELOG.md, records a structured release plan, and opens a fully validated release PR. Automate label-driven release-branch backports, including conflict handoff and release-branch commit validation.

Acceptance criteria

• One RC and one final release are prepared through the workflow.
• At least one label-driven backport is completed, including validation that all selected backports merged.
• Generated notes require only curated Highlights and Upgrading edits.
• Manual version editing and cherry-pick instructions are replaced, with a temporary break-glass path retained until phase 18.
• The implementation PR is linked and merged.

[DariuszPorowski]

Implementation PR: #12823 (ci: automate release preparation and backports), submitted as PR 12 in stack #12738 on top of #12793.