Fix grouping by ruby version

Matching by managers was missing "gitlabci" since 1ccd7042. This
should be less fragile.

renovate.json

@@ -6,12 +6,9 @@
   ],
   "packageRules": [
     {
-      "matchManagers": [
-        "asdf",
-        "bundler",
-        "devcontainer",
-        "dockerfile",
-        "custom.regex"
+      "matchDatasources": [
+        "ruby-version",
+        "docker"
       ],
       "matchPackageNames": [
         "ruby",