Revert "[root7] Root7 only compiles with C++17."
Revert "[root7] Root7 only compiles with C++17."

This reverts commit 5558580e1fe093f2e3c1fbfdaf8f592090678368.

I was mislead because of the old ROOT-performance-centos7-multicore/default bot
telling me it could not build in -DCMAKE_CXX_STANDARD=14.

karies/root,karies/root,root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,karies/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root