Disable dependabot for version updates that aren't security related.

This will still allow dependabot security updates according to https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#open-pull-requests-limit.

PiperOrigin-RevId: 677879732

.github/dependabot.yml

@@ -4,5 +4,5 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
-    # Allow up to 3 opened pull requests for github-actions versions
-    open-pull-requests-limit: 3
+    # Don't allow non-security PRs to be opened.
+    open-pull-requests-limit: 0