Reposition epics on the board using graphQL

What does this MR do?

We want to be able to reposition epics on the epic board lists.

This MR refactors issues-related services (MoveService and UpdateService) to add the support for that.

Another MR to change the mutation will follow.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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  • 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 #323194 (closed)