Fix code scanning alert no. 9: Use of insecure HostKeyCallback implementation

[yarlson]

Fixes https://github.com/yarlson/ftl/security/code-scanning/9

To fix the problem, we need to replace the insecure ssh.InsecureIgnoreHostKey() with a secure host key callback. The best way to do this is to use the ssh.FixedHostKey function, which validates the host key against a predefined allow list. This ensures that only trusted host keys are accepted.

1. Read the allowed host key from a file (e.g., allowed_hostkey.pub).
2. Parse the public key.
3. Use ssh.FixedHostKey with the parsed public key to set the HostKeyCallback.

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