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: /messages/by-id/e0f09c9a-bd2b-862a-d379-601dfabc8969@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a0b762626884b3b949c2703abb1c4b42fbbdfdc6

Modified Files
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doc/src/sgml/release.sgml | 60 ++---------------------------------------------
src/tools/RELEASE_CHANGES | 6 ++---
2 files changed, 4 insertions(+), 62 deletions(-)