rmv social slack channel from list as it has been archived

Why is this change being made?

Issue - https://gitlab.com/gitlab-com/marketing/marketing-operations/issues/1569 - requested to update the Social & Twitter slack channels to combine them.

This is to consolidate ways to get intouch with the social team.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

/close https://gitlab.com/gitlab-com/marketing/marketing-operations/issues/1569