Remove 9.6 (#298)

* Drop EOL postgres 9.6

* Drop individual READMEs

Checking the history the individual READMEs used to provide that
version specific info and it no longer does.

Author: BryanQuigley

.github/workflows/main.yml

@@ -15,7 +15,7 @@ jobs:
   make-docker-images:
     strategy:
       matrix:
-        postgres: [9.6, 10, 11, 12, 13, 14]
+        postgres: [10, 11, 12, 13, 14]
         postgis: ['2.5', '3.2']
         variant: [default, alpine]
         exclude: