[cmake] Refactored MetisSetup.cmake.

• Use target instead of global properties.
• Use metis mirror at https://gitlab.opengeosys.org/ogs/libs/metis.git to properly set #define IDXTYPEWIDTH to 64.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Test this with a complete workflow (partmesh -> sim -> check results); unfortunately we have no ctest for this yet.