Simplify release-note links to back branches.

Now that https://www.postgresql.org/docs/release/ is populated,
replace the stopgap text we had under "Prior Releases" with
a pointer to that archive.

Discussion: https://postgr.es/m/e0f09c9a-bd2b-862a-d379-601dfabc8969@postgresql.org

doc/src/sgml/release.sgml

@@ -86,64 +86,8 @@ For new features, add links to the documentation sections.
   <title>Prior Releases</title>
 
   <para>
-   Release notes for prior release branches can be found on the
-   <ulink url="https://www.postgresql.org/"><productname>PostgreSQL</productname></ulink>
-   web site.  At the time of release of version 12,
-   these were the supported prior release branches:
-
-   <itemizedlist>
-    <listitem>
-     <para>
-      <productname>PostgreSQL</productname> 11:
-      <ulink url="https://www.postgresql.org/docs/11/release.html">
-       <literal>https://www.postgresql.org/docs/11/release.html</literal>
-      </ulink>
-     </para>
-    </listitem>
-
-    <listitem>
-     <para>
-      <productname>PostgreSQL</productname> 10:
-      <ulink url="https://www.postgresql.org/docs/10/release.html">
-       <literal>https://www.postgresql.org/docs/10/release.html</literal>
-      </ulink>
-     </para>
-    </listitem>
-
-    <listitem>
-     <para>
-      <productname>PostgreSQL</productname> 9.6:
-      <ulink url="https://www.postgresql.org/docs/9.6/release.html">
-       <literal>https://www.postgresql.org/docs/9.6/release.html</literal>
-      </ulink>
-     </para>
-    </listitem>
-
-    <listitem>
-     <para>
-      <productname>PostgreSQL</productname> 9.5:
-      <ulink url="https://www.postgresql.org/docs/9.5/release.html">
-       <literal>https://www.postgresql.org/docs/9.5/release.html</literal>
-      </ulink>
-     </para>
-    </listitem>
-
-    <listitem>
-     <para>
-      <productname>PostgreSQL</productname> 9.4:
-      <ulink url="https://www.postgresql.org/docs/9.4/release.html">
-       <literal>https://www.postgresql.org/docs/9.4/release.html</literal>
-      </ulink>
-     </para>
-    </listitem>
-   </itemizedlist>
-  </para>
-
-  <para>
-   Release notes for older release branches can be found at
-   <ulink url="https://www.postgresql.org/docs/manuals/archive/">
-    <literal>https://www.postgresql.org/docs/manuals/archive/</literal>
-   </ulink>
+   Release notes for prior release branches can be found at
+   <ulink url="https://www.postgresql.org/docs/release/"><literal>https://www.postgresql.org/docs/release/</literal></ulink>
   </para>
  </sect1>
 

src/tools/RELEASE_CHANGES

@@ -124,10 +124,8 @@ Creating Back-Branch Release Notes
 Retiring a Branch
 =================
 
-After support is dropped for an old branch, remove the link for it
-in HEAD's doc/src/sgml/release.sgml.  If the newest back branch hasn't
-yet reached GA release, do the same there.  (Given our current development
-calendar, the latter should seldom be necessary.)
+* Get the buildfarm's 'branches_of_interest.txt' file updated to remove
+  the retired branch.