What's the CI task to develop? Please describe

If users wish to contribute to the chart, they should do so in the longhorn/longhorn repository instead of the longhorn/charts repository, as the charts repository is exclusively used for release purposes.

Describe the items of the task development (DoD, definition of done) you'd like

Add a GitHub action to close the PRs created by external contributors and leave a comment directing them to contribute to the longhorn/longhorn repository instead.

Additional context

cc @yangchiu @derekbit