Add meeting cleanup day template

Adds Slack template wording for meeting cleanup day

Also updates to post in #whats-happening-at-gitlab

Closes gitlab-com/ceo-chief-of-staff-team/cos-team#352

content/handbook/ceo/chief-of-staff-team/_index.md

@@ -297,7 +297,7 @@ When changing the cadence of a meeting, a team member can copy and paste this me
 
 If you are a team member who intends to decline a meeting, the [asynchronous communication](/handbook/company/culture/all-remote/asynchronous/#how-to-decline-meetings-in-favor-of-async) section of the handbook has some good suggestions for what to say when you decline.
 
-Meeting Cleanup Day is intentionally near the start of the new fiscal year. The CoST will launch this initiative annually a week in advance through posting in the `#whats-happening-at-gitlab` Slack channel, and by re-posting a reminder at 00:00 UTC on cleanup day. CoST (or CoS) will repost in the managers channel for visibility.
+Meeting Cleanup Day is intentionally near the start of the new fiscal year. The CoST will launch this initiative annually a week in advance through posting in the `#whats-happening-at-gitlab` Slack channel, and by re-posting a reminder at 00:00 UTC on cleanup day. CoST (or CoS) will repost in the `#managers` channel for visibility.
 
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