remove rsync support

rsync was executed as an external command, which means we have no insight
into or control over what it actually does.
From a security perspective, this is far from ideal.

To be clear, there's nothing inherently wrong with rsync itself. However,
if we were to support it properly within SFTPGo, we would need to implement
the low-level protocol internally rather than relying on launching an external
process. This would ensure it works seamlessly with any storage backend,
just as SFTP does, for example.
We recommend using one of the many alternatives that rely on the SFTP
protocol, such as rclone

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

openapi/openapi.yaml

@@ -5385,7 +5385,7 @@ components:
         max_upload_file_size:
           type: integer
           format: int64
-          description: 'maximum allowed size, as bytes, for a single file upload. The upload will be aborted if/when the size of the file being sent exceeds this limit. 0 means unlimited. This restriction does not apply for SSH system commands such as `git` and `rsync`'
+          description: 'maximum allowed size, as bytes, for a single file upload. The upload will be aborted if/when the size of the file being sent exceeds this limit. 0 means unlimited'
         tls_username:
           type: string
           description: 'defines the TLS certificate field to use as username. For FTP clients it must match the name provided using the "USER" command. For WebDAV, if no username is provided, the CN will be used as username. For WebDAV clients it must match the implicit or provided username. Ignored if mutual TLS is disabled. Currently the only supported value is `CommonName`'