feat: add a relaxed list of FIPS default algorithms

The FIPS-supported algorithms added in
!207 may be too
restrictive. FIPS-supported algorithms may not be backwards compatible
with existing clients, such as those that are configured to use the
ssh-rsa (SHA-1 hash) key exchange algorithm.

This commit adds a `ssh.DefaultAlgorithms()` that returns mostly the
previous defaults used in FIPS, minus the elliptical curve algorithms,
which panic on the Go 1.24 FIPS compiler.