Review/merge .com agent and engineer onboarding

As we are no longer hiring the agent role, the two onboarding templates need to be merged and reviewed:

• gitlab-dot-com focus is up to date: https://gitlab.com/gitlab-com/support/support-training/-/blob/master/.gitlab/issue_templates/Onboarding%20-%20GitLab-dot-com%20Focus.md (was agent onboarding issue)
• Application Support focus is out of date but has a bunch of pieces that we want to keep: https://gitlab.com/gitlab-com/support/support-training/-/blob/master/.gitlab/issue_templates/DEPRECATED%20-%20Onboarding%20-%20Application%20Support%20Focus.md (was '.com engineer' issue template)

@lyle I believe you had someone working on this. Could you please re-assign this to them? If not, I'm fairly sure I could do it?