Drop obsolete patch root-add-flexiblas-detection.patch (cmake's
FindBLAS.cmake supports flexiblas now)
Drop the workaround for the bug in doxygen causing different results
on 32 and 64 bit architectures (use doxygen < 1.8.17 or >= 1.8.20-3)
Build require xrootd 5 (Fedora 33+, EPEL 7+)