Deprecate Slack slash commands

Thiago Figueirórequested to merge
569345-deprecate-slack-slash-commands into master

What does this MR do and why?

Deprecate Slack slash commands.

The why is explained in https://gitlab.com/gitlab-com/Product/-/work_items/14352+.

References

Deprecate Slack Slash Commands (#569345) • Unassigned • 19.0

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