Expose epic web_url as URL

What does this MR do?

Exposes epics web_url as full URL instead of path on API to be consistent with issues and merge requests.

This fixes a QA failure: #33115 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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