pgsql: doc PG 18 relnotes: update to current

doc PG 18 relnotes: update to current

Backpatch-through: 18 only

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a60691eb201dfaa8a2c3aec8815d08ed3371aae7

Modified Files
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doc/src/sgml/release-18.sgml | 25 +++++++++++++++++++++----
1 file changed, 21 insertions(+), 4 deletions(-)