ssh: support rsa-sha2-256/512 for client certificates

The server-sig-algs logic was not working for certificate algorithms.
Follow-up on CL 392394.

Tested with OpenSSH 8.8 configured with

    PubkeyAcceptedKeyTypes -ssh-rsa-cert-v01@openssh.com

Updates golang/go#39885
For golang/go#49952

Change-Id: Ic230dd6f98e96b7938acbd0128ab37d33b70abe5
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/392974
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>