matrix-synapse: default to postgresql on 18.03

nixos/doc/manual/release-notes/rl-1803.xml

···
       <literal>lib.mkOverride</literal> can be used.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      The following changes apply if the <literal>stateVersion</literal> is changed to 18.03 or higher.
+      For <literal>stateVersion = "17.09"</literal> or lower the old behavior is preserved.
+    </para>
+    <itemizedlist>
+      <listitem>
+        <para>
+          <literal>matrix-synapse</literal> uses postgresql by default instead of sqlite.
+          Migration instructions can be found <link xlink:href="https://github.com/matrix-org/synapse/blob/master/docs/postgres.rst#porting-from-sqlite"> here </link>.
+        </para>
+      </listitem>
+    </itemizedlist>
+  </listitem>
 </itemizedlist>
 
 </section>

nixos/modules/services/misc/matrix-synapse.nix

···
       };
       database_type = mkOption {
         type = types.enum [ "sqlite3" "psycopg2" ];
-        default = "sqlite3";
+        default = if versionAtLeast config.system.stateVersion "18.03"
+          then "psycopg2"
+          else "sqlite3";
         description = ''
           The database engine name. Can be sqlite or psycopg2.
         '';