chore: use centralized renovate configuration

References https://github.com/gradlex-org/renovate-config so we can
change renovate defaults for all repositories in a central place.
Uses json5 syntax which is easier to write than JSON, see
https://json5.org/.

Author: britter

.github/renovate.json5

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