pgsql: Simplify release-note links to back branches.

Simplify release-note links to back branches.

Now that https://www.postgresql.org/docs/release/ is populated,
replace the stopgap text we had under "Prior Releases" with
a pointer to that archive.

Discussion: https://postgr.es/m/e0f09c9a-bd2b-862a-d379-601dfabc8969@postgresql.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b94181ffb2a552ee5cd7223e00590c106d64907d

Modified Files
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doc/src/sgml/release.sgml | 60 ++---------------------------------------------
1 file changed, 2 insertions(+), 58 deletions(-)