Doc: update our claims about the minimum recommended AIX version.

We currently have buildfarm members testing back to AIX 7.1,
but not before, and older AIX versions are long out of support
from IBM.  So say that 7.1 is the oldest supported version.

Discussion: https://postgr.es/m/87y278s6iq.fsf@wibble.ilmari.org

doc/src/sgml/installation.sgml

@@ -2174,9 +2174,15 @@ export MANPATH
    </indexterm>
 
    <para>
-    PostgreSQL works on AIX, but AIX versions before about 6.1 have
+    You can use GCC or the native IBM compiler <command>xlc</command>
-    various issues and are not recommended.
+    to build <productname>PostgreSQL</productname>
-    You can use GCC or the native IBM compiler <command>xlc</command>.
+    on <productname>AIX</productname>.
+   </para>
+
+   <para>
+    <productname>AIX</productname> versions before 7.1 are no longer
+    tested nor supported by the <productname>PostgreSQL</productname>
+    community.
    </para>
 
    <sect3>