Update distros on the docs start page

Centos 7 is no longer supported, replace with Centos 8. Also Debian
hasn't been working for some time and progress in fixing it has stalled,
so drop it for now.

Change-Id: Ic1513b20f296978bca095c7aa79f022d7d9ab7ac
Closes-Bug: 1881183

Author: jharbott

doc/source/index.rst

@@ -38,8 +38,7 @@ Install Linux
 
 Start with a clean and minimal install of a Linux system. DevStack
 attempts to support the two latest LTS releases of Ubuntu, the
-latest/current Fedora version, CentOS/RHEL 7, as well as Debian and
-OpenSUSE.
+latest/current Fedora version, CentOS/RHEL 8 and OpenSUSE.
 
 If you do not have a preference, Ubuntu 18.04 (Bionic Beaver) is the
 most tested, and will probably go the smoothest.