Migrated issue spec files to Jest

What does this MR do?

Tests has been updated to run with Jest, additionally the IssueMilestone has been optimized to use computed valute rather than data since those are not updated when props change, but only on component init.

Fixes: #58827 (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

Components using the IssueMilestone component should be manually verified to work since there was changes made. I have tested this locally on my machine and looks good.

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