Resolve "Migrate Bootstrap button to GitLab UI GlButton in app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_merged.vue"

What does this MR do?

Migrates the Bootstrap buttons to GitLab UI GlButton in app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_merged.vue

Screenshots

	Before	After
Resting state
Hover revert button
Hover cherry-pick button
Hover delete source branch button

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #229298 (closed)