Use External Data Checker to automate PRs for new versions

I noticed 4.10.5 got released with a feature i want, so i made #106, patterned off of #105 since it seemed like previous PRs were maybe manually created.

As someone working on maintaining a flatpak myself, I wanted to see if https://docs.flathub.org/docs/for-app-authors/external-data-checker would be helpful for automating this process - it (to my understanding) checks hourly for new sources (i.e. versions of webcord) and will automatically create PRs for them.

This may help reduce the number of flatpak users who have the experience I just did, where webcord says "hey there's an update" but `flatpak update` doesn't have the update available (granted its only been like a day since the release, so maybe the benefit here is marginal anyway)