Allow epic tree GraphQL mutation to change parent

What does this MR do?

Modifies EpicTree::Reorder to also take an optional new_parent_id to modify the parent epic (or epic for an EpicIssue).

Adds auth to ensure the epic can be moved from the old parent to the new, and calls existing validations to ensure the new parent is valid.

Backend portion to !28629 (merged).

Related to #33039 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team