Find7z: make it work also on non-Windows systems

https://cmake.org/cmake/help/latest/command/find_program.html does not mention anything about .exe, but looking around no find_program call seems to do special things for Windows extensions. So they seem not needed. And removing them makes it work on Linux & Co. as well.

The current exe extensions have been used because when introducing this module in !158 (merged) 7z was thought of only as Windows-variant of the gzip approach used for compressing SVGs during build. But people might want to use /z also on Linux, because it might do better compression in some situations.

Using 5.113 in note, because planning to backport.