Roll out the new Slack GBP app in the internal GitLab workspace

Background

Initial notes from &7729 (comment 875803434):

Roll out the GBP app in the GitLab workspace.

• Announce this to our own team members, in case they encounter any breakage.
• This would mean updating the Classic app in our workspace to GBP, and at this point any needed changes in GitLab should already be tested and deployed.
• As part of this step we'd also test the slash commands functionality again, and see if we need any further changes.
• We'll also see what exactly each workspace needs to do once we rollout the GBP app to all gitlab.com users, so we'll probably document this for later use.

Proposal

Update the Slack app in the GitLab workspace to GBP. This will only affect our own workspace, i.e. the "development" version of this app and not the marketplace listing.

We should announce to team members that we're doing this and let them know to contact us if they encounter any breakage.

The process for the update should be the same as when we updated one of the Classic testing apps to GBP in #355994 (closed), but now we'll be doing it with the production app https://api.slack.com/apps/A676ADMV5. See notes at #355994 (comment 921061361).

The slash command should continue working, both after updating the settings and reinstalling the app.

To reinstall the app in the Slack workspace we might need the help of Slack admin at GitLab.

Availability & Testing

Suggestion: Regression against the GitLab ecosystem workspace.