Fix removing labels when moving an epic between lists

What does this MR do?

It makes sure that labels are correctly removed when moving between epic board lists. The problem was, that wen moving an epic from the labeled list to close/open list the list labels were not removed. That was caused by the fact that the parent class methods were used and they were built for issues boards only.

I fixed that by rewriting those method for epic boards and overriding the service method.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • [-] I have not included a changelog entry because _____.
• [-] 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.

Related to #326461 (closed)