DOCSP-21104 drop EOL OSes (#678)

Author: kanchana-mongodb

source/install/compatibility.txt

@@ -40,7 +40,7 @@ systems:
      - 2012, 2012R2, 2016, 2019
      - ``x86-64``
    * - Red Hat Enterprise Linux / CentOS
-     - 6, 7, 8
+     - 7, 8
      - ``x86-64``, ``ARM``
    * - SUSE Linux Enterprise Server
      - 12, 15