[18.6] Revert update to sshd_config mandating stronger algorithms

What does this MR do and why?

This backports !8937 (merged) to 18-6-stable.

This reverts merge request !8806 (merged) because it breaks existing installations. There should be a way for users to set these settings.

Related issues

Relates to #9483

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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