Updated CMake version requirement in build.rst (#7487)

The documentation states that to build from source you need CMake 3.13 or higher. However, according to https://github.com/dmlc/xgboost/blob/master/CMakeLists.txt#L1 CMake 3.14 or higher is required.

doc/build.rst

@@ -52,7 +52,7 @@ This shared library is used by different language bindings (with some additions
 on the binding you choose).  The minimal building requirement is
 
 - A recent C++ compiler supporting C++11 (g++-5.0 or higher)
-- CMake 3.13 or higher.
+- CMake 3.14 or higher.
 
 For a list of CMake options like GPU support, see ``#-- Options`` in CMakeLists.txt on top
 level of source tree.