Increase Dependabot open-pull-requests-limit to 20

The default limit of 5 caused Dependabot to stop creating new PRs
when CI-failing PRs accumulated without being closed, blocking
major version updates for @actions/core and @actions/github.

Author: ChanTsune

.github/dependabot.yml

@@ -9,6 +9,7 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+    open-pull-requests-limit: 20
     groups:
       jest:
         patterns: