SSH security.

- Replace the RSA key for GitHub with an ed25519 one.
- Refactor the security configuration in 99_default. Make them easier to
  maintain by sepcifying what *not* to use.
- In the OpenSSH 8.5 release notes it is mentioned that the ssh-rsa
  signature scheme will be deperecated. Be proactive and disable it.
- Replace the RSA key for *.shore.co.il with a ed25519 one (keep to the
  ecdsa so as not to just a single key). Keep it for ea6350, Dropbear in
  OpenWRT 19.07 doesn't support other key types.