pgsql: Replace search.cpan.org with metacpan.org

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

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c91ac358b190cf51ec0334215987f6633f289544

Modified Files
--------------
doc/src/sgml/acronyms.sgml          | 2 +-
doc/src/sgml/external-projects.sgml | 2 +-
doc/src/sgml/install-windows.sgml   | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)