doc: update release note item about the v2 wire protocol

Protocol v2 was last used in PG 7.3, not 7.2.

Reported-by: Tatsuo Ishii

Discussion: https://postgr.es/m/20210608.091329.906837606658882674.t-ishii@sraoss.co.jp

doc/src/sgml/release-14.sgml

@@ -213,7 +213,7 @@ Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
      </para>
 
      <para>
-      This was last used as the default in Postgres 7.2 (year 2002).
+      This was last used as the default in Postgres 7.3 (year 2002).
      </para>
     </listitem>