Use Renovate instead of Dependabot (#1736)

Dependabot has not detected plugin BOM releases for several weeks.
It seems to also ignore plugin version numbers generated by automated
releases.

More details in the pull requests linked from:

* jenkinsci/versioncolumn-plugin#402

### Testing to be done

* After merging the pull request, I will find the plugin in the Renovate
  installed repositories (https://developer.mend.io/github/jenkinsci) and
  run "Actions" / "Run Renovate scan" for the plugin.  That will create
  the Dependency Dashboard issue for the plugin

* I will pin the Dependency Dashboard issue and click the checkbox to
  allow all pending pull requests to be created

Author: MarkEWaite

.github/dependabot.yml

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "github>jenkinsci/renovate-config"
+  ]
+}