After years of forewarning, disable the RSA/SHA-1 signature algorithm

After years of forewarning, disable the RSA/SHA-1 signature algorithm
by default. It is feasible to create colliding SHA1 hashes, so we
need to deprecate its use.

RSA/SHA-256/512 remains available and will be transparently selected
instead of RSA/SHA1 for most SSH servers released in the last five+
years. There is no need to regenerate RSA keys.

The use of RSA/SHA1 can be re-enabled by adding "ssh-rsa" to the
PubkeyAcceptedAlgorithms directives on the client and server.

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