Aligns with v0.x release flow. Can delete long-lived release branch after merging.

Summary by cubic

Switch v1.x release flow to CI-created, per-version branches. Retires the long‑lived release branch and aligns with v0.x.

Description

• Replaces deprecated ::set-output with $GITHUB_OUTPUT; writes newTag and newBranch from package.json version.
• Creates release-v1x/version-$VERSION and opens PRs to v1.x via peter-evans/create-pull-request.
• Synced with the latest v1.x to avoid drift.

Docs

• Update any docs/automation that reference the release branch and plan to delete it after merge.

Testing

• No unit tests needed.
• Validate by running the workflow and confirming a PR from release-v1x/version-$VERSION to v1.x with the correct tag and branch name in outputs.

^{Written for commit d427573. Summary will update on new commits.}