chore: deprecate CODER_SSH_HOSTNAME_PREFIX in favor of CODER_WORKSPACE_HOSTNAME_SUFFIX (#21836)

## Description

Mark `--ssh-hostname-prefix` flag and `CODER_SSH_HOSTNAME_PREFIX` env
variable as deprecated, recommending users to use
`--workspace-hostname-suffix` / `CODER_WORKSPACE_HOSTNAME_SUFFIX`
instead for consistency with Coder Desktop.

The deprecated option is now hidden from help output and docs but
remains functional for backward compatibility. When used, it will show a
deprecation warning pointing to the recommended alternative.

## Changes

- Added `UseInstead` pointing to `workspace-hostname-suffix` option
(triggers deprecation warning)
- Set `Hidden: true` to hide from CLI help and documentation
- Updated description to mention deprecation
- Regenerated docs and help files via `make gen`

Closes #18156

---

_Originally requested by @matifali in
https://github.com/coder/coder/pull/18085#discussion_r2115594447_

docs/reference/cli/server.md

@@ -1198,17 +1198,6 @@ Disable password authentication. This is recommended for security purposes in pr
 
 Specify a YAML file to load configuration from.
 
-### --ssh-hostname-prefix
-
-|             |                                         |
-|-------------|-----------------------------------------|
-| Type        | <code>string</code>                     |
-| Environment | <code>$CODER_SSH_HOSTNAME_PREFIX</code> |
-| YAML        | <code>client.sshHostnamePrefix</code>   |
-| Default     | <code>coder.</code>                     |
-
-The SSH deployment prefix is used in the Host of the ssh config.
-
 ### --workspace-hostname-suffix
 
 |             |                                               |