Use a correct group path when fetching epic in graphql boards

What does this MR do?

The MR requires :graphql_board_lists FF and concerns project and group issue boards in EE.

Resolves #328564 (closed).

(It's best to follow the reproduction steps outlined in the issue to test the feature).

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 the affected feature is behind a non-default FF.
• 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.

  => Testing at a feature level is required but this will be handled in a separate MR.

• 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

Related to #328564 (closed)