Add API trigger to "Check Certificates" workflow

The `repository_dispatch` event allows triggering workflows via the GitHub API. This might be useful for triggering an
immediate check in multiple relevant repositories after an external change, or some automated process. Although we don't
have any specific need for this event at the moment, the event has no impact on the workflow, so there is no reason
against having it. It is the sort of thing that can end up being useful if it is already in consistently in place, but
not worth setting up on demand, since the effort to set it up is greater than the effort to trigger all the workflows
manually.

.github/workflows/check-certificates.yml

@@ -1,13 +1,12 @@
 name: Check Certificates
 
+# See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
 on:
   schedule:
     # run every 10 hours
     - cron: "0 */10 * * *"
-  # workflow_dispatch event allows the workflow to be triggered manually.
-  # This could be used to run an immediate check after updating certificate secrets.
-  # See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows#workflow_dispatch
   workflow_dispatch:
+  repository_dispatch:
 
 env:
   # Begin notifications when there are less than this many days remaining before expiration