Prevent just anyone from skipping credits but allow admins

https://git.magicalgirl.moe/STJr/SRB2/merge_requests/476#note_11225

This fixes the former of the two issues that I mentioned.

I forget if I asked already on Discord, but is this actually netsafe with 2.1.23? Given how this affects how the XD_EXITLEVEL net command works, it's hard for me to tell.

changed the description

changed target branch from master to next

Whoops. It's not, in the case that you join a server not running this branch and as an admin try skipping the credits--it basically has the old behavior of letting you skip it without the server reflecting so.

merged

mentioned in commit 755e4a8b