`priority:*` labels in the github repository are becoming `severity:*`,
and `frequency:*` — `reach:*`. This commit will update the slack
notifications for the first responders accordingly.

Also, delete the github-label-issues-to-triage.py one-off script, which
worked with the old label names and is by now too distant in the past to
be useful as reference or anything.

Release Notes:

- N/A