ROOT: Update to 5.34.11 for 10.9 support and add HEAD support

This commit bumps ROOT to version 5.34.11.

It also adds a HEAD URL/branch for additional patches to the current
ROOT release.  The HEAD branch points to the *official* ROOT GitHub
mirror instead of the CERN git server simply because the CERN git server
is primarily for internal use.  While HEAD support is not strictly
necessary for 10.9, adding HEAD will allow users access to more 10.9
patches (since support is still incomplete) before the next ROOT
release.