[core] Limit targetfinding vertical to 8/8.5y

[sruon]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

I had documented it when researching various distance mechanics.
Retested today, still seems to hold true.

8y cap on most actions except mob self-centered which is 8.5y.

Supposedly some zones, possibly Dynamis have a different limit but I tested Dreamlands and it lined up with expectations.

Steps to test these changes

• Cast a buff with P2 at 7.999 and 8.000
• !fafnir and have P2 test various vertical coords with Spike Flail (P1 centered, 8y) and Hurricane Wing (Fafnir centered, 8.5y)