Fix improper xreflabels created for v18 release notes

Looking at https://www.postgresql.org/docs/current/xplang.html during
a near by discussion, I was bothered that plpython was not formatting
the same way as the other PL's listed on that page, so I went
spelunking to see what had happened.

It turns out that some xref labels were added during this commit
(https://github.com/postgres/postgres/commit/d8aa21b74ff4e3d767c3344484c3cb22b9f0ec0d)
in order to make the links more readable in the v18 release notes.

However, based on the discussion here:
https://www.postgresql.org/message-id/20200611223836.GA2507%40momjian.us,
the correct way to style links from the release notes is through the
<link> tag, not by adding xrefs, which can have unintended
side-effects in other places (like in the initial doc link above).

This is a little tricky to fix as it needs to happen across 2
different branches, but I believe the right way to fix this involves 2
parts. First, apply patch 01, which converts the xrefs in the v18
release notes to link tags, against REL_18_STABLE, since those release
notes don't exist on master.

Second, either revert the original commit linked above, or apply the
patch 02 which just undoes those changes. Note, I created the patch
against master, but I think it should be applied there and back
patched to 18 stable.

Also note I did a quick scan to see if either the liboq or plpython
xreflabel situation existed elsewhere but I only turned up one usage,
at https://www.postgresql.org/docs/current/triggers.html, which is
also corrected with these changes.

Robert Treat
https://xzilla.net