Use Renovate instead of Dependabot

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"
+  ]
+}