Provide clearer message when users link Story from non next-gen project with Epic of next-gen project

Summary

When users attempt to link a Story of non next-Gen project with Epic of Next-Gen project, Jira still shows them in the list of possible Epics to be links. However, since Epic from next-Gen project has no Epic Name, it shows as unlabelled-issue key and has below error.

Issue 'issue key' must be of type 'Epic'.

From users' perspective, this is Epic issue type from next-gen project and they don't expect this to be much difference from other Epics. With the above error message returned, it is confusing users.

Steps to reproduce

1. Have 2 projects (Next-gen and non next-gen)
2. Create an Epic from Next-gen project
3. Create a Story from non next-gen project
4. Edit the Story and try to link it from Edit Issue screen
5. Update

Expected Result

This seems not currently possible to do this crossed project linking between Next-gen and non next-gen project. Below are the proposed solution/expected behavior:

1. Update our documentation about this limitation
2. Epics from Next-gen projects should not be returned in the list of Epics, when users attempt to link it from non Next-gen project
3. Provide clearer error message that users can't link with Epic from Next-gen project

Actual Result

Below error message is shown: