Add tooltip for "Squash commits" option when at disabled state

The following discussion from !33827 (merged) should be addressed:

• @pedroms started a discussion:

  @sming-gitlab ⚙ Behavior: Unfortunately, one aspect I forgot to mention was that, if the “squash” checkbox is visible and disabled (when Require is selected) we need to provide some explanation to users why it's checked and disabled. Let me know if this is feasible in this MR or if we should separate that to another MR.

  MR form	MR widget
  Because there's enough space, we can always display the explanation.	Because space is limited, we defer the explanation to a tooltip.