ssh: use RSA/SHA-256 instead of RSA/SHA-1 as the public key algorithm

openssh now disables RSA/SHA-1 by default, so using RSA/SHA-1 will
eventually cause us problems:

https://undeadly.org/cgi?action=article;sid=20210830113413

in addition, github will disable RSA/SHA-1 for recently added RSA keys:

https://github.blog/2021-09-01-improving-git-protocol-security-github/

this patch modifies ssh.c to use RSA/SHA-256 (aka rsa-sha2-256)
instead of RSA/SHA-1 (aka ssh-rsa) as the public key algorithm.

NOTE: public rsa keys and thumbprints are ***NOT AFFECTED***
by this patch.

while we're here, remove the workaround for github.com. it seems
that github has fixed their implementation, and does not look into
macalgs when we're using an aead cipher.
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