Remove deprecated_clipboard_button in Slack slash commands

What does this MR do and why?

Replace with clipboard_button. Part of work needed to wrap up #417796 (closed).

Changelog: changed

Screenshots or screen recordings

Before	After

How to set up and validate locally

Go to Project > Settings > Integrations > Slack slash commands. For example, http://127.0.0.1:3000/gitlab-org/gitlab-test/-/settings/integrations/slack_slash_commands/edit.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.