Bump the required cmake version to `3.10.2`
Bump the required cmake version to `3.10.2`

CMake `3.9` introduced the library target `OpenMP::OpenMP_C` which is
way more portable than the old compiler flag based approach. Due to the
required version bump we surveyed the available CMake versions on widely
used OS distributions. Considering 3.10.2 is oldest suported in
Ubuntu 18.04 LTS (EOL april 2023): https://packages.ubuntu.com/search?keywords=cmake
we choose `3.10.2` as the new required version which only excludes RHEL7.

https://lists.llvm.org/pipermail/llvm-dev/2020-April/140578.html

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