Update extensions-CheckUser.php

This duplicates patchdemo!97 (merged) for catalyst backend wikis.

Why:

The CheckUser extension implements a onboarding dialog for temporary accounts that is shown to users with rights to reveal their IPs This can be annoying for test wikis, as the user does not need to be informed how to use the tools Hiding the dialog by default on patch demo wikis avoids the issue, while still allowing testing the dialog at a later time

What:

Set $wgDefaultUserOptions['checkuser-temporary-accounts-onboarding-dialog-seen'] = 1; in for the LocalSettings file for CheckUser.

Bug: T385708