crypto/elliptic: deprecate unsafe APIs

crypto/elliptic: deprecate unsafe APIs

Per the updated go.dev/wiki/Deprecated, those APIs replaced by
crypto/ecdh (added in Go 1.20) can now be marked as deprecated
in Go 1.21.

Updates #52221
Updates #34648

Change-Id: Id0e11d7faa3a58a1716ce1ec6e2fff97bab96259
Reviewed-on: https://go-review.googlesource.com/c/go/+/459977
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>