Deprecate the default id_ed25519_lagoon_cli.key key in cli images.

[rocketeerbkw]

Lagoon offers the ability to inject the project private key as a build arg, and allows users to inject any arbitrary key as an environment variable.

Providing a default keypair was only needed when users didn't have other options, so I think we can remove it. We can deprecate it and make sure it's not included when new major versioned images are released.

It exists in the node-cli and php-cli images.