Migrate to GlTooltipDirective in mr_widget_author.vue

What does this MR do?

• For &1079 (closed) >> &973
• For technical-writing#215 (closed)
• Replace tooltip with GlTooltipDirective in /app/assets/javascripts/vue_merge_request_widget/components/mr_widget_author.vue

Closes #228974 (closed)

Screenshots (strongly suggested)

I'm not sure where is this tooltip in the UI but I suppose it should be in one of these:

Tooltip over the author name when MR is closed:

BEFORE	AFTER
Only title	Tooltip
author_closed_mr_before	author_closed_mr_after

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team