Use Ruby version syntax for Dependabot-Docker

According to
https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference#versions-ignore
this is needed. Unfortunately I can find very little information
on "Ruby version syntax", but I think this comment states that
removing the `.x` might do it:
dependabot/dependabot-core#2644 (comment)

.github/dependabot.yml

@@ -73,4 +73,4 @@ updates:
     ignore:
       - dependency-name: "node"
         # Odd-numbered versions are unstable releases, so skip those.
-        versions: ["23.x", "25.x", "27.x", "29.x"]
+        versions: ["23", "25", "27", "29"]