Doc: Update ulinks to RFC documents to avoid redirect

Daniel Gustafsson <dgustafsson@postgresql.org>

Commit: ef124d0952e18f1c7a7e9f99b2d10e2f0737c24a
Author: Daniel Gustafsson <dgustafsson@postgresql.org>
Date: 2024-04-10T11:53:25Z
Releases: 15.7
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

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