Add option to disable writing to authorized_keys file

What does this MR do?

Allows us (and customers) to stop writing to the authorized_keys file since we don't use it.

Why was this MR needed?

The authorized_keys file is already not used, so it is wasteful and unexpected to continue writing changes to it.

Screenshots

Application Settings

Documentation for configuring sshd to use GitLab Shell's authorized_keys command

Note: The setting's help icon links to the Tidying Up heading.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #1631 (closed)