[Chapel Merge] Document that C++14 is required in prereqs.rst

Branch: refs/heads/master
Revision: da4ad68
Author: mppf
Log Message:

Merge pull request #17604 from mppf/doc-c++-14

Document that C++14 is required in prereqs.rst

Resolves #10525

This PR updates prereqs.rst to indicate that the chpl requires C++14 in order to build.
We are moving to LLVM-by-default and LLVM has required C++14 since version 9.

Reviewed by @vasslitvinov and @gbtitus - thanks!

Modified Files:
M doc/rst/usingchapel/prereqs.rst

Compare: https://github.com/chapel-lang/chapel/compare/136ef20581c9...da4ad6838ca6