Spike: Investigate other needed code changes when upgrading the Slack app to GBP

Background

Initial notes from &7729 (comment 875803434):

Spike: Check for any other breakage with the GBP app.

• Test the slash commands functionality, see if everything still works correctly.
• This might result in more follow-up issues, which we probably need to address before moving on.

Proposal

This is a placeholder issue to check for any breakage after upgrading the Slack app to GBP, and investigate if further code changes are needed beyond the known ones in #355997 (closed)

We should test all user interactions with the app:

• Installing the app.
• Reinstalling the app.
• Uninstalling the app.
• Test the help command (no authorization needed).
• Test a different command like issue show <id> which triggers the authorization flow.
  • All the commands except help go through the same general code path, so we don't necessarily need to test every one of them.
• Test the authorization flow.

For any problems that we encounter we should create follow-up issues, and depending on severity they should be blocking the rollout of the upgraded app in #355999 (closed).

Available commands

The following commands are available for this chatops integration:

/gitlab [project name or alias] issue show <id>
/gitlab [project name or alias] issue new <title> ⇧ Shift+↵ Enter <description>
/gitlab [project name or alias] issue search <your query>
/gitlab [project name or alias] issue move <issue_id> (to)? <project_path>
/gitlab [project name or alias] issue close <id>
/gitlab [project name or alias] issue comment <id> ⇧ Shift+↵ Enter <comment>
/gitlab [project name or alias] deploy <environment> to <target-environment>
/gitlab [project name or alias] run <command> <arguments>

If available, the run command is used for running GitLab CI jobs
defined in this project's .gitlab-ci.yml file. For example, if a
job called "help" is defined you can run it like so:

/gitlab [project name or alias] run help