feat: add a relaxed list of FIPS default algorithms

The FIPS-supported algorithms added in !207 (merged) 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.