Updated the emoji picker component default button slot

What does this MR do and why?

Updated the emoji picker component default button slot

• Updated the default button slot in the emoji picker component to match the most common use-case.
• Updated the files that used that use-case to use the default slot.

This dries up the codebase somewhat and reduces some of the non-compliant button findings

Screenshots or screen recordings

emoji-picker.mov

How to set up and validate locally

1. Go to an issue in your GDK
2. Upload a design to that issue
3. Click on the design to add a comment / discussion item.
4. You should see the emoji picker on the comment you made (see screen recording)
5. Add a regular comment on that issue
6. You should see the same emoji picker on that comment
7. Add a task to that issue
8. Add a comment on that task
9. Enable the work-items-mvc2 feature flag
10. You should see the emoji picker on that comment.

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.

Related to #418229 (closed)