Externals Update, 21.0-mc16a branch (2020.10.07.)

Updated all projects to atlasexternals-1.0.27.3. It includes just one update wrt. atlasexternals-1.0.27.2 (atlasexternals@1.0.27.2...1.0.27.3): It stops AtlasLCG from setting up an RPM dependency for packages that the given LCG release doesn't actually provide. (But the package is still found somewhere else. For instance under /usr.)

This is necessary to fix the "latest" issue on ATLINFR-3741.

Note that I will start a full build of Athena myself now as well, to test whether I would be able to install it in the same way in which the nightly is supposed to. So let's not merge this in until I have some results from that...