Update versions of ROOT and FairRoot

If possible, update version of ROOT to the latest one (6.26/02) and together with this also version of FairRoot to 18.6.8. The FairRoot needs to be forked from the original repository and memory leak patch must be applied. Also default tree name should be changed from cbmsim to mpdsim in order to remove rootmanager.dat file. Change of tree name will be done by sed script during the build process (and not during patching of FairRoot). All changes in versions should be done preferably via defaults-release.sh.