Add Find7Zip, deprecate Find7z, revert Find7z broken Linux support
p7zip has been an old fork for Linux of the original 7-Zip program, and occupying the 7z namespace, though just supporting the very 7z format, other than the original. When 7-zip started to also support Linux, it had to adapt to that and used 7zz as binary name.
Using 7(-)Zip instead of 7z is also more correct when talking about the actual 7-Zip program.
Plan is to backport to ECM 5, which already got the (not yet released) backport of the broken 7z Linux support.
Matches also Craft's approach -> https://invent.kde.org/packaging/craft/-/blob/master/bin/utils.py?ref_type=heads#L57
Motivated by games/libkdegames@0b3708cf (comment 815224)