update to ROOT v6-28-12

[KrisThielemans]

hopefully fixes #857

[KrisThielemans]

v6-28-12 compiled TBB ok, but the failed in XROOTD installation. XROOTD-install-err.log says

[I] No git repository info found. Trying to interpret VERSION_INFO
[I] The XrdVersion.hh file already exists
gmake[7]: *** read jobs pipe: Bad file descriptor.  Stop.
gmake[7]: *** Waiting for unfinished jobs....
gmake[6]: *** [Makefile:136: all] Error 2
CMake Error at /home/runner/work/SIRF-SuperBuild/SIRF-SuperBuild/build/builds/ROOT/build/builtins/xrootd/XROOTD-prefix/src/XROOTD-stamp/XROOTD-install-Release-impl.cmake:9 (message):
  Command failed (2):

   '/usr/local/bin/cmake' '--build' '.' '--target' 'install'

[KrisThielemans]

Needed more recent `nlohmann/json due to root-project/root#14634

Still stuck due to #861

[KrisThielemans]

Although #857 auto-disappeared (presumably due to a wrong config of the GHA runners), updating ROOT is necessary for C++-17, which will be required for the next STIR version (UCL/STIR#1236 in progress), so best to merge this now.