Migrate from Dependabot to Renovate

[matt-richardson]

Migrates dependency updates from Dependabot to the self-hosted Renovate setup used across team-builds repositories.

Changes

• Add renovate.json (house config: dependencies/autoapprove labels, prConcurrentLimit: 1, minimumReleaseAge: 3 days). Includes the dependency dashboard (Issues enabled on this repo).
• Add .github/workflows/renovate.yaml (daily runner; uses the RENOVATE_GITHUB_TOKEN repo secret).
• Add .github/workflows/renovate-approve-pr.yaml (auto-approves team-builds-bot PRs labelled autoapprove, then auto-merges once required status checks pass).
• Remove .github/dependabot.yml (nuget ecosystem).
• Remove .github/workflows/dependabot-automerge.yml.

Notes

• Renovate auto-detects package files, so the previous per-directory/per-ecosystem Dependabot config is not needed.
• Prerequisite: this repo needs a RENOVATE_GITHUB_TOKEN repo-level secret before the runner will work — not yet set, needs an admin to add it.
• If Renovate has opened its own default "Configure Renovate" onboarding PR on this repo, close it — this PR supersedes it.

Part of the team-builds Dependabot → Renovate migration (DPT-3076).

🤖 Generated with Claude Code

[matt-richardson]

This PR's branch was deleted by our own Renovate Check Dependencies job (RENOVATE_BASE_BRANCHES bug, see new PR for details) -- not intentionally closed. Recreated, with the underlying bug fixed, as a new PR.