Release v14.57.6

Prepares for the v14.57.6

• Sign Cells Shell JWT per request !1570 (merged)
• Update module github.com/mattn/go-shellwords to v1.0.15 !1565 (merged)

Notes for the maintainer

• v14.57.5 hasn't been tagged yet. !1567 (merged) (Release v14.57.5) was merged on 2026-09-23 as 51001424, but no v14.57.5 tag exists; the latest tag is v14.57.4. Please tag v14.57.5 on 51001424 as well, so the CHANGELOG and tags stay consistent.
• After merging, tag v14.57.6 on the merge commit, then bump GITLAB_SHELL_VERSION in gitlab-org/gitlab to 14.57.6.
• Why now: !1570 (merged) blocks the Cells SSH routing production rollout. Without it, a git push whose Git LFS pre-push hook takes longer than ~60s fails with cells routing: receive-pack response: Not found. Staging validation: #874 (comment 3894763329). Change request: gitlab-com/gl-infra/production#22944.

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