doc: PG 16 relnotes, update for beta 2

Bruce Momjian <bruce@momjian.us>

Commit: 228a2fab9cdee8bad434a0037e8f5e85c6f30002
Author: Bruce Momjian <bruce@momjian.us>
Date: 2023-06-26T17:44:40Z
Releases: 16.0
doc:  PG 16 relnotes, update for beta 2

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