Fixed buttons styles for mattermost slash commands integration

What does this MR do and why?

Fixes styles for the copy clipboard button under the Mattermost slack commands project integration

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Navigate to Project > Settings > Integrations > Mattermost slack commands

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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