Upgrade CMake min ver to 3.13.

[nitrix]

What

This upgrades the CMake minimum version to 3.13.

Why

Version 3.0 is no longer supported by newer toolchains and fails to build.

Exhibit

[cmake] CMake Error at vendor/enet/CMakeLists.txt:1 (cmake_minimum_required):
[cmake]   Compatibility with CMake < 3.5 has been removed from CMake.
[cmake] 
[cmake]   Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
[cmake]   to tell CMake that the project requires at least <min> but has been updated
[cmake]   to work with policies introduced by <max> or earlier.
[cmake] 
[cmake]   Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
[cmake] 
[cmake] 
[cmake] -- Configuring incomplete, errors occurred!

Considerations

While upgrading to something barely above 3.5+ to satisfy the requirements would work, there has been a similar upgrade done recently in CGLM (see recp/cglm#430) from 3.8.2 to 3.13.0 and provides a lot more context why specifically 3.13.0 is more desirable. The defaults with set()/option() are less surprising and don't risk breaking backwards compatibility as something far newer could.