coreos-base/coreos-init: SSHD: use secure crypto algos only

[t-lo]

This change updates coreos-init to a version which includes
a new SSHD config to limit crypto to "known secure" algorithms only.

See flatcar/init#36 for details.

Note that I'll update this PR to point to the merge commit of the source PR when that one gets merged. Bringing up this PR early so it can be reviewed in parallel to the source PR.

Testing done

$ emerge-amd64-usr coreos-init
$ ./build_image --board=amd64-usr
$ ./image_to_vm.sh ...

Started QEmu image, SSH'd into image, then

core@localhost ~ $ sudo cat /etc/ssh/sshd_config 
# Use most defaults for sshd configuration.
Subsystem sftp internal-sftp
ClientAliveInterval 180
UseDNS no
UsePAM yes
PrintLastLog no # handled by PAM
PrintMotd no # handled by PAM
Ciphers chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha2-256,hmac-sha2-512,umac-128-etm@openssh.com,umac-128@openssh.com
KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256

Impact

While this change is not expected to have a noticeable impact to the vast majority of users, some ancient clients which only offer (by today's standards) insecure ciphers may be affected. We need to raise this in our release communication.

[dongsupark]

ok if CI passed