Fix "Merge when pipeline succeeds" button dropdown caret icon horizontal alignment

What does this MR do?

Fix "Merge when pipeline succeeds" button dropdown caret icon horizontal alignment

Before	After

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The dropdown caret icon had some extra general style margin-left applied which offset from center.

It looks like this regressed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12601 when we switched from fa-caret-down to fa-chevron-down which has some general margin styles applying

• https://gitlab.com/gitlab-org/gitlab-ce/blame/cbaa015cc9f55a387cdab85a6ba4b8c9c6ab447e/app/assets/stylesheets/framework/buttons.scss#L216
• https://gitlab.com/gitlab-org/gitlab-ce/blame/cbaa015cc9f55a387cdab85a6ba4b8c9c6ab447e/app/assets/stylesheets/framework/dropdowns.scss#L105

Before (rough revert)	After

https://gitlab.com/gitlab-org/gitlab-ce/issues/37194#note_38823599

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37194 (closed)