Replace search.cpan.org with metacpan.org

search.cpan.org has been EOL'd, with metacpan.org being the official
replacement to which URLs now redirect.  Update links to match the new
URL. Also update links to CPAN to use https as it will redirect from
http.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/B74C0219-6BA9-46E1-A524-5B9E8CD3BDB3@yesql.se

doc/src/sgml/acronyms.sgml

@@ -71,7 +71,7 @@
     <term><acronym>CPAN</acronym></term>
     <listitem>
      <para>
-      <ulink url="http://www.cpan.org/">Comprehensive Perl Archive Network</ulink>
+      <ulink url="https://www.cpan.org/">Comprehensive Perl Archive Network</ulink>
      </para>
     </listitem>
    </varlistentry>

doc/src/sgml/external-projects.sgml

@@ -65,7 +65,7 @@
       <entry>DBD::Pg</entry>
       <entry>Perl</entry>
       <entry>Perl DBI driver</entry>
-      <entry><ulink url="http://search.cpan.org/dist/DBD-Pg/"></ulink></entry>
+      <entry><ulink url="https://metacpan.org/release/DBD-Pg/"></ulink></entry>
      </row>
 
      <row>