crypto/ecdsa: deprecate direct use of big.Int fields in keys

crypto/ecdsa: deprecate direct use of big.Int fields in keys

Updates #63963

Change-Id: I6a6a69645e625cde1ac1c6abf698a5fd3d52b4cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/701516
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>