_content/doc: add Go 1.21 release notes

Copied from the main Go repo at CL 515955.
Deleted there in CL 516075.

For golang/go#58645.

Change-Id: If91f782f6429a53cbc52e0e37fbad88fb80db006
Reviewed-on: https://go-review.googlesource.com/c/website/+/516095
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>