Disable Dependabot version updates

This allows security updates to still get created.

This behavior is documented at
https://docs.github.com/en/code-security/dependabot/dependabot-security-updates/configuring-dependabot-security-updates#overriding-the-default-behavior-with-a-configuration-file

.github/dependabot.yml

@@ -4,4 +4,5 @@ updates:
   directory: "/"
   schedule:
     interval: daily
-  open-pull-requests-limit: 10
+  # disable version updates
+  open-pull-requests-limit: 0