worldborder set and add have a maximum of 59999968 rather than 60000000

The worldborder set and worldborder add commands have a maximum of 59999968 rather than 60000000.

Steps to reproduce:

• Type /worldborder set 60000000 or /worldborder add 60000000 in chat.
• An error gets shown saying that the double must not be more than 59999968.