Fix the return type of UUIDMigration#getIdentificationFor(Player)

Because this method returned a String instead of a UUID, Pet#getOwner()
did not work properly: `ownerIdentification` was always a String as of
[Pet.java:92](https://github.com/DSH105/EchoPet/blob/5a2b59b1cec6ac91a18e566b6b5e50db5f5d495a/modules/EchoPet/src/main/java/com/dsh105/echopet/api/pet/Pet.java#L92)