Externals Update, 21.0 branch (2020.10.06.)

Updated all projects to atlasexternals-1.0.48.6. The changes wrt. atlasexternals-1.0.48.5 are (atlasexternals@1.0.48.5...1.0.48.6):

• Stopped FindDoxygen.cmake from setting up Pango and Cairo automatically for the runtime environment;
  • This was a cherry-pick from the master and 1.0 branches of atlas/atlasexternals from a while ago;
• Fixed the RPM dependency setup of FindCairo.cmake;
  • I learned it just now that the RPM/package changed its name from Cairo to cairo between LCG_91 and LCG_92...

This is necessary to make the RPMs built for the x86_64-centos7-gcc62-opt platform installable.

Pinging @jchapman. 😉