doc/go1.20: add release notes for net/http and net/http/httputil

doc/go1.20: add release notes for net/http and net/http/httputil

For #41773
For #41773
For #50465
For #51914
For #53002
For #53896
For #53960
For #54136
For #54299

Change-Id: I729d5eafc1940d5706f980882a08ece1f69bb42c
Reviewed-on: https://go-review.googlesource.com/c/go/+/450515
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>