Roll out the new Slack GBP app to all gitlab.com users

Background

Initial notes from &7729 (comment 875803434):

Roll out the GBP app to all gitlab.com users.

• Announce the update, including the needed steps for workspace admins based on what we learned so far.
• Get the updated app published in the marketplace.
• We probably also want to track this in our usage data, so we can see how many users have updated.

Proposal

This will involve a few steps:

• Announce the update to our Slack app (maybe in a release post), including a link to the documentation we created as part of #355999 (closed).
• Test the GitLab (dev) Slack app against staging.
• Submit the updated production app to the marketplace, maybe coordinate with Slack to get a timeline for this.
• Be ready for possible support requests, documentation tweaks, bug fixes.