Question: drop the up-to-date rule and enable CircleCI auto-cancel #315

Problem

On this strict-base rebase line (rebase merges, never a force push) every merge to the base sends every open PR back through a full CI run on a fresh branch: devctl pr wait exits 3 with "behind giantswarm, which requires branches to be up to date", and since the branch cannot be rebased in place, the PR is replaced by a new one from a fresh branch of the base, with a full rerun of the 42 checks (about 30 minutes). One two-commit change needed three PRs (#400, #402, #405) and three CI runs because #392, then #394 and #403 merged ahead of it. With the workspaces order on this repository the cost is one full CI run per open PR per merge.

Also, CircleCI's redundant-workflow auto-cancel is off for the project, so pipelines of superseded heads hold queue slots.

Proposed solution

A decision for the team, no change by the reporting worker. Two options for the up-to-date requirement:

1. A merge queue for the giantswarm branch. GitHub tests each PR on top of the base plus the PRs ahead of it and merges it without a replacement PR. The check stays strict; the CI pipeline has to run on the queue's merge_group refs, and the merge gate has to enqueue instead of merging.
2. Drop the up-to-date requirement for the giantswarm branch in the repository's ruleset declaration in giantswarm/github. A PR merges as soon as its own CI is green; the risk is a semantic conflict between two PRs that each passed alone, caught only on the base afterwards.

Independently: turn on auto-cancel of redundant workflows in the CircleCI project settings (the owners hold them).

Acceptance criteria

• The team picks one option for the up-to-date requirement; after it lands, a merge to the base no longer turns another open PR into a replacement PR.
• Pipelines of superseded heads are cancelled automatically.

Written by an agent.