[winswitch] RHEL / CentOS 8 and the clones

[Antoine Martin]

Hi,

The xpra build system can now generate packages for most of the clones / 
derived distributions:
* OracleLinux
* AlmaLinux
* RockyLinux
* CentOS stream8
The last two xpra releases (4.3.4 and 3.1.3) included packages for all 
of the above, including some arm64 builds.
The first three do generate identical binaries which is comforting.

For the time being, the "RedHat" download link continues to be a 
symbolic link to the CentOS packages.
The problem with this is that there are no CentOS 8.6 builds.
At present, the packages we rely on in RHEL 8.6 are still ABI compatible 
with 8.5 so this is not a big problem, yet.
So rather than choosing which distribution to use as a replacement for 
the now defunct CentOS 8, I have left things unchanged. But this may 
have to be changed when 8.7 (or later) is released.
Which clone should replace CentOS as the "one true clone" is not clear 
at this point and would only be changed for the ">=8.6" series and 
perhaps also "9.x", but not the "7" and older builds.
(making the repository link maintenance more difficult than it already is)

If you have suggestions or opinions on what should be done, feel free to 
chime in - here, on IRC or on github issues.

Cheers,
Antoine