doc: move Go 1.21 release notes to x/website

Now that the development of the Go 1.21 release is almost done, its
release notes are moved to their eventual long-term home in x/website
in CL 516095. Delete the initial development copy here.

For #58645.

Change-Id: I5207d21289b2e7b9328c943a088f45bc81c710a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/516075
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>