reflect: deprecate SliceHeader and StringHeader

reflect: deprecate SliceHeader and StringHeader

These are replaced by unsafe.String etc, which were added in Go 1.20.
Per https://go.dev/wiki/Deprecated, we must wait until Go 1.21
to mark them deprecated.

Fixes #56906.

Change-Id: I4198c3f3456e9e2031f6c7232842e187e6448892
Reviewed-on: https://go-review.googlesource.com/c/go/+/452762
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Russ Cox <rsc@golang.org>