Set minimum CMake version to 3.14.5

[akoeplinger]

See discussion in 16e1d30#r396389297

The documented minimum CMake version is 3.15.5 already but a few places still check for 3.14:

$ git grep "_minimum_required\|3\.14\.2"  # search for _minimum_required OR 3.14.2

docs/workflow/requirements/linux-requirements.md:Building the repo requires CMake 3.14.2 or newer on Linux. Add Kitware's APT feed to your configuration for a newer version of CMake. See their instructions at <https://apt.kitware.com/>. Also, add LLVM/s APT feed to your configuration for a newer version of CMake. See their instructions as <http://apt.llvm.org/>.
eng/native/build-commons.sh:    if [[ "$(version "$cmake_version")" -lt "$(version 3.14.2)" ]]; then
eng/native/build-commons.sh:        echo "Please install CMake 3.14.2 or newer from http://www.cmake.org/download/ or https://apt.kitware.com and ensure it is on your path."; exit 1;
global.json:    "cmake": "3.14.2",
src/coreclr/CMakeLists.txt:cmake_minimum_required(VERSION 3.14.2)
src/coreclr/src/pal/tests/CMakeLists.txt:cmake_minimum_required(VERSION 3.14.2)
src/coreclr/tests/CMakeLists.txt:cmake_minimum_required(VERSION 3.14.2)
src/installer/corehost/CMakeLists.txt:cmake_minimum_required(VERSION 3.14.2)
src/installer/corehost/cli/test/mockhostfxr/CMakeLists.txt:cmake_minimum_required (VERSION 2.6)
src/libraries/Native/Unix/CMakeLists.txt:cmake_minimum_required(VERSION 3.14.2)
src/libraries/Native/Windows/CMakeLists.txt:cmake_minimum_required(VERSION 3.14.2)
src/libraries/Native/Windows/clrcompression/zlib/CMakeLists.txt:cmake_minimum_required(VERSION 2.4.4)
src/mono/CMakeLists.txt:cmake_minimum_required (VERSION 3.10)
src/mono/netcore/sample/iOS/CMakeLists.txt:cmake_minimum_required(VERSION 3.14.5)

[vcsjones]

@jkoritzinsky wasn't there an issue with Alpine that prevented bumping the minimum CMake version to match documentation?

[jkoritzinsky]

There was. We can't use 3.15.5 on Alpine because packages for Alpine 3.11 aren't backward-compatible with Alpine 3.10 and older. However, Alpine 3.10 ships with CMake 3.14.5, so this is doable.