doc: Remove link to JSON support in the SQL specification

The link used in the documentation is dead, and the only options to have
an access to this part of the SQL specification are not free.  Like any
other books referred, just remove the link to keep some neutrality but
keep its reference.

Reported-by: Erik Rijkers
Discussion: https://postgr.es/m/989abd7d-af30-ab52-1201-bf0b4f33b872@xs4all.nl
Backpatch-through: 12

Author: michaelpq

doc/src/sgml/biblio.sgml

@@ -137,7 +137,7 @@
    </biblioentry>
 
    <biblioentry id="sqltr-19075-6">
-    <title><ulink url="https://standards.iso.org/ittf/PubliclyAvailableStandards/c067367_ISO_IEC_TR_19075-6_2017.zip">SQL Technical Report</ulink></title>
+    <title>SQL Technical Report</title>
     <subtitle>Part 6: SQL support for JavaScript Object
       Notation (JSON)</subtitle>
     <edition>First Edition</edition>