Update ROOT version detection mechanism
Update ROOT version detection mechanism as discussed in ATLINFR-5075 for the dev3 nightlies.
The ROOT version numbers moved from https://github.com/root-project/root/blob/master/core/foundation/inc/RVersion.h to https://github.com/root-project/root/blob/master/core/foundation/inc/ROOT/RVersion.hxx in the ROOT master branch.
Need to keep the previous mechanism since the v6-28-00-patches branch used for the dev4 nightlies is unchanged.
Tag @akraszna for feedback if this change looks reasonable.