doc: Update SQL features/conformance information to SQL:2023

Peter Eisentraut <peter@eisentraut.org>

Commit: c9f57541d9705da44b9fbcb6e0c75dec3b41b57c
Author: Peter Eisentraut <peter@eisentraut.org>
Date: 2023-04-05T07:20:25Z
Releases: 16.0
doc: Update SQL features/conformance information to SQL:2023

Optional subfeatures have been changed to top-level features, so there
is a bit of a churn in the list for that.

Some existing functions have been added to the standard, so they are
moved from the "other" to the "standard" lists in their sections.

Discussion: https://www.postgresql.org/message-id/flat/63f285d9-4ec8-0c9e-4bf5-e76334ddc0af@enterprisedb.com

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