Make offboarding info for developers more specific

.gitlab/issue_templates/Offboarding.md:
Add more specific information for developer offboarding (e.g. also
remove from arch-dev mailing list).
Add more specific information on which templates to choose and where to
open a ticket for signing key removal.
2 jobs for extend_offboarding_for_devs in 1 minute and 13 seconds (queued for 3 seconds)