chore: update Renovate preset reference to converged path

Andrew Dunnrequested to merge
renovate-converge-preset into main

What

Update the shared-preset reference to gitlab>gitlab-com/public-sector/pipeline:renovate-config.

Why

The previous reference used the subdirectory file path (pipeline//presets/renovate.json), which is a compatibility shim. This points the project at the canonical preset location directly, keeping the estate aligned on a single source of truth for Renovate governance. Any project-specific rules are preserved unchanged.

Validation

Validated with renovate-config-validator --strict.

DO NOT MERGE pending human review.

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