Fixes merge request widget date tooltip inconsistencies

What does this MR do?

Previously the merged request widget merged & closed states where using the merge requests updated_at date which is incorrect. This date is updated whenever the merge request is updated through comments, description changes etc.

Now the merged & closed states use their own event date which includes their own updated_at dates. These dates are also correctly formatted through our gl.utils date utility methods to create a user friendly tooltip text.

Screenshots (if relevant)

Before	After

What are the relevant issue numbers?

Closes #38545 (closed)