feat: group renovate github actions updates (#60)

Let Renovate group GitHub Actions updates.

Author: jooola

renovate/default.json

@@ -10,6 +10,11 @@
   "minimumReleaseAge": "5 days",
   "minimumReleaseAgeBehavior": "timestamp-optional",
   "packageRules": [
+    {
+      "description": ["group github actions updates"],
+      "groupName": "github actions",
+      "matchManagers": ["github-actions"]
+    },
     {
       "description": ["update actions weekly"],
       "matchManagers": ["github-actions"],