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_

cli/testdata/coder_server_--help.golden

@@ -215,9 +215,6 @@ Clients include the Coder CLI, Coder Desktop, IDE extensions, and the web UI.
           commas.Using this incorrectly can break SSH to your deployment, use
           cautiously.
 
-      --ssh-hostname-prefix string, $CODER_SSH_HOSTNAME_PREFIX (default: coder.)
-          The SSH deployment prefix is used in the Host of the ssh config.
-
       --web-terminal-renderer string, $CODER_WEB_TERMINAL_RENDERER (default: canvas)
           The renderer to use when opening a web terminal. Valid values are
           'canvas', 'webgl', or 'dom'.