…10438)

This commit is preparation for the infrastructure to be used when
supporting [Wasmtime LTS releases][rfc]. The goal here is to add some
automation and infrastructure to perform a weekly build of all active
release branches which will file an issue on failure. This should
ideally keep branches up-to-date and ensure that we don't forget to
backport any fixes to older branches. Or rather when we do forget to
backport fixes this'll be a reminder to go do that anyway.

The general architecture here is:

* A new `ci-cron-trigger.yml` workflow is added.
* This new workflow runs once-a-week and runs a small script that
  triggers CI for all active release branches.
* The main CI, `main.yml`, is updated to file an issue on failure when
  triggered in this fashion.

While I was here I additionally removed the `schedule:` from the
`main.yml` to instead fold the daily scheduling of CI runs into this new
script as well. That way all our cron CI jobs are gated in workflows
that require this exact repository meaning that forks won't be running
cron jobs.

[rfc]: bytecodealliance/rfcs#42