Fix validate-renovate workflow path drift (.json5 → .json)

[adsurg]

What

The validate-renovate.yml workflow is wired to a filename that doesn't exist in this repo, so the Renovate config has effectively been unvalidated on PRs.

Two mismatches in .github/workflows/validate-renovate.yml:

1. paths: filter targeted .github/renovate.json5, but the on-disk config is .github/renovate.json. PRs touching the config never triggered the workflow.
2. Validator step used pattern: '*.json5', which matches no files in the repo.

Both now point at .github/renovate.json, the actual config filename.

Why this direction (.json rather than renaming to .json5)

.json5 is Renovate's preferred format, but the existing renovate.json uses no JSON5-only syntax (no comments, no trailing commas, no unquoted keys). Renaming would be a stylistic migration with no functional benefit and a larger diff. The smaller, safer fix is to align the workflow with the file that already exists.

Verifying it runs

Touched .github/renovate.json with a no-op "description" field so this PR exercises the paths: filter and you can see the workflow trigger and pass green. The field is semantically inert for Renovate; it just documents that the file is validated on PR.

Scope

• No behavioural change to Renovate itself — same extends, same packageRules.
• Out of scope: any other rule/schedule changes to the Renovate config.
• Noted in passing (not fixed here): the per-project review also flagged this drift alongside other small CI hygiene items in the repo — happy to follow up separately if useful.

Context

Came out of a project-level repo review rather than a GitHub issue. Companion small-fix PR to backstage/charts #326 (the dual-stack Helm-doc README fix), part of the same review pass.