Create Github action to automatically import schemas from Firefox

[Rob--W]

Our most recent schema imports (#5385, #5386) were delayed because these are manual tasks and it fell through the cracks while we were busy.

We should automatically create the issue and PR on a schedule, e.g. via Github actions' schedule: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule

It is possible to automatically determine the upcoming branch/release dates via: https://whattrainisitnow.com/

We could run an import 1-2 week after a release reaches Beta.