Update offboarding checklist for staff members

Author: rrrutledge

.github/ISSUE_TEMPLATE/offboard-staff.md

@@ -7,4 +7,7 @@ assignees: rrrutledge
 ---
 
 - [ ] Remove from [staff team](https://github.com/orgs/InnerSourceCommons/teams/staff).
+- [ ] Remove from [#innersource-marketing-team](https://app.slack.com/client/T04PXKRM0/C05KNQT26RK).
+- [ ] Remove from [#flax-and-teal-isc](https://app.slack.com/client/T04PXKRM0/C04K4K6362C).
+- [ ] Remove from weekly staff meeting.
 - [ ] Decommission `@innersourcecommons.org` email.