pgsql: Doc: Update ulinks to RFC documents to avoid redirect

Doc: Update ulinks to RFC documents to avoid redirect

The tools.ietf.org site has been decommissioned and replaced by a
number of sites serving various purposes.  Links to RFCs and BCPs
are now 301 redirected to their new respective IETF sites.  Since
this serves no purpose and only adds network overhead, update our
links to the new locations.

Backpatch to all supported versions.

Discussion: https://postgr.es/m/3C1CEA99-FCED-447D-9858-5A579B4C6687@yesql.se
Backpatch-through: v12

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bfed70500289fae9e14f5535d1a1d237ad45d8fd

Modified Files
--------------
doc/src/sgml/charset.sgml      | 2 +-
doc/src/sgml/client-auth.sgml  | 4 ++--
doc/src/sgml/json.sgml         | 2 +-
doc/src/sgml/libpq.sgml        | 2 +-
doc/src/sgml/ref/psql-ref.sgml | 2 +-
5 files changed, 6 insertions(+), 6 deletions(-)