Migrate dependency updates from Dependabot to Renovate with auto-merge

[erichaagdev]

Replaces Dependabot with the Mend-hosted Renovate app, with auto-merge enabled so routine dependency PRs no longer require manual review.

Changes

• Add renovate.json (extends config:recommended, automerge: true, platformAutomerge: true, dependency dashboard on).
• Delete .github/dependabot.yml.

Behavior after this lands

• One PR per dependency (Gradle deps, Gradle wrapper, and GitHub Actions are all covered by Renovate's default managers).
• Any update — patch, minor, or major — auto-merges once the Build check is green; a red build (e.g. a breaking bump like openapi-generator 7.22.0) leaves the PR open for manual attention.
• No release "bake" delay — updates merge as soon as they're green.
• The dummy gradle-plugin-portal registry Dependabot required is no longer needed (Renovate resolves the plugin portal natively).

Required out-of-band steps (not in this PR)

• Branch protection on main requiring the Build check (admins exempt) + "Allow auto-merge" enabled — configured via API alongside this PR.
• Install/authorize the Renovate GitHub App on the repo (manual, owner-only).

This PR itself dogfoods the setup: it is configured to auto-merge once Build passes.