chore: Switch from dependabot to renovate

This allows much better configuration and customization and will keep our
release branches up2date, too.

.github/renovate.json

@@ -0,0 +1,16 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:best-practices"
+  ],
+  "baseBranches": [
+    "main",
+    "/^v[0-9]+/"
+  ],
+  "packageRules": [
+    {
+      "matchBaseBranches": [ "/^v[0-9]+/" ],
+      "groupName": "all dependencies"
+    }
+  ]
+}