pgsql: Doc: in each release branch,keep only that branch's own release

Doc: in each release branch, keep only that branch's own release notes.

Historically we've had each release branch include all prior branches'
notes, including minor-release changes, back to the beginning of the
project.  That's basically an O(N^2) proposition, and it was starting to
catch up with us: as of HEAD the back-branch release notes alone accounted
for nearly 30% of the documentation.  While there's certainly some value
in easy access to back-branch notes, this is getting out of hand.

Hence, switch over to the rule that each branch contains only its own
release notes.  So as to not make older notes too hard to find, each
branch will provide URLs for the immediately preceding branches'
release notes on the project website.

There might be value in providing aggregated notes across all branches
somewhere on the website, but that's a task for another day.

Discussion: https://postgr.es/m/cbd4aeb5-2d9c-8b84-e968-9e09393d4c83@postgresql.org

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0d13dd371b64c52221ba220d214d8fbce3e474f5

Modified Files
--------------
doc/src/sgml/filelist.sgml    |    15 -
doc/src/sgml/release-10.sgml  |  9330 --------------------------
doc/src/sgml/release-7.4.sgml |  4622 -------------
doc/src/sgml/release-8.0.sgml |  5421 ---------------
doc/src/sgml/release-8.1.sgml |  5444 ---------------
doc/src/sgml/release-8.2.sgml |  7077 -------------------
doc/src/sgml/release-8.3.sgml |  8549 -----------------------
doc/src/sgml/release-8.4.sgml | 10080 ----------------------------
doc/src/sgml/release-9.0.sgml | 11091 ------------------------------
doc/src/sgml/release-9.1.sgml | 11761 --------------------------------
doc/src/sgml/release-9.2.sgml | 12201 ---------------------------------
doc/src/sgml/release-9.3.sgml | 14551 ----------------------------------------
doc/src/sgml/release-9.4.sgml | 13159 ------------------------------------
doc/src/sgml/release-9.5.sgml | 11192 ------------------------------
doc/src/sgml/release-9.6.sgml | 11341 -------------------------------
doc/src/sgml/release-old.sgml |  6557 ------------------
doc/src/sgml/release.sgml     |    93 +-
17 files changed, 74 insertions(+), 142410 deletions(-)