chore: adopt shared Public Sector Renovate preset

Andrew Dunnrequested to merge
renovate-converge-preset into main

What

Replace the generic config:recommended base with the shared Public Sector preset gitlab>gitlab-com/public-sector/pipeline:renovate-config.

Why

This brings the project onto the estate-wide single source of truth for Renovate governance, so dependency policy is managed centrally rather than diverging per project. The project-specific rule that disables project-level cosign pins is preserved.

Validation

Validated with renovate-config-validator --strict.

DO NOT MERGE pending human review.

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