chore(deps): replace Dependabot with Renovate

Adds renovate.json modelled after the kubernetes repo config.
Removes .github/dependabot.yml.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

.github/dependabot.yml

@@ -1,19 +0,0 @@
-version: 2
-updates:
-  # Maintain dependencies for GitHub Actions
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    labels:
-      - "dependencies"
-      - "github-actions"
-
-  # Maintain dependencies for Docker
-  - package-ecosystem: "docker"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    labels:
-      - "dependencies"
-      - "docker"