Disable dependabot version updates

We use Renovate for this, but still want dependabot security scanning so
we configure 0 pull requests:

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuring-dependabot-version-updates#disabling-dependabot-version-updates

.github/dependabot.yml

@@ -4,3 +4,4 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+    open-pull-requests-limit: 0