Prevent major ruby docker updates in renovate.json (google#4907)

Updated `renovate.json` to prevent major updates for the `ruby` docker
image.
This is in response to PR google#4894
trying to upgrade ruby from 3 to 4.
The affected file is likely `docs/docs.Dockerfile` which uses `FROM
ruby:3`.


---
*PR created automatically by Jules for task
[2175686184353370820](https://jules.google.com/task/2175686184353370820)
started by @another-rex*

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: 2 people

renovate.json

@@ -137,6 +137,18 @@
       ],
       "enabled": false
     },
+    {
+      "matchDatasources": [
+        "docker"
+      ],
+      "matchPackageNames": [
+        "ruby"
+      ],
+      "matchUpdateTypes": [
+        "major"
+      ],
+      "enabled": false
+    },
     {
       "matchDatasources": [
         "docker"