[MOD] Switching new Unified Admin panel to be off by default.

Tiki24 LTS, like all LTS versions should be as stable as possible. Tiki25 will see a huge amount of changes: Bootstrap 5, Smarty 4, Rubix ML 2, support for PHP 8.0 and 8.1, and many more at https://dev.tiki.org/Tiki25

A lot of work has gone into the new Unified Admin Panel. But quite a bit of work and testing remains to be done. At the meeting today https://tiki.org/Roundtable-Meeting-2022-01, there were some discussions/debates which should not be done at the time of the release process. These important discussions (like should admin panel be themeable, what goes in admin panels for full admins vs site management tools for junior admins, etc.) should have been sorted out (one way or another) outside of the release process.

So this merge request is to make the new admin panel default off in Tiki24. I am hopeful it can not only be on by default in Tiki25 (that is current code in main/trunk), but we should not have two admin panels, as this is more code/complexity to maintain. I hope Tiki25 will be the new admin panel, themable with Bootstrap 5, like everything else in Tiki.

Please join https://dev.tiki.org/Administration-UX-and-Control-Panels-re-organization