Platform Name Updates, master branch (2023.03.31.)

Tweaked the OS name finding code, so that now it would handle /etc/os-release files on Linux better.

For now the LCG platform name is kept as "centos" on all EL-like platforms by default. So in LCG nightlies (like dev3) where the x86_64-el9-gcc11-opt platform name already exists, it will need to be forced explicitly in the build system for now.

What I wanted to be more provocative with is the naming that we would choose for the ATLAS releases themselves. In this MR, on purpose, I chose a naming that would hopefully be controversial enough to trigger a discussion. 😛 As it would assign "centos", "alma" and "rhel" names separately for different EL flavours. (And I just now remembered that I should've added Rocky Linux as well...) I don't think we should be doing this, but I thought I'd get better reactions like this. 😛

So... is it time that we would switch to a new platform naming scheme? 🤔 Note that the LCG and ATLAS platform names do not need to go hand-in-hand. The default chosen for the offline and TDAQ releases do go hand-in-hand though. (It's possible to override them individually, but their defaults are chosen by the same CMake code.) So there we should move to the el naming in sync. 😉

Pinging @elmsheus, @fwinkl, @rhauser, @aundrus, @boeriu.