sftpgo: mark as unfreeRedistributable

Resolves #315154.

New Web UI uses an unfree web theme, and as a result it...

> is allowed for use only within the SFTPGo product and therefore
> cannot be used in derivative works/products without an explicit
> grant from the SFTPGo Team ([e-mail]).

This makes the entire package unfree, which the [SFTPGo license
compliance page][1] supports:

> If you modify SFTPGo's source code and are therefore creating a
> derivative work:
>
> 1. ...
> 2. ...
> 3. You cannot use the UI theme based on KeenThemes because it is
> based on a proprietary theme that we purchased and the WebAdmin and
> WebClient components created using this theme can only be used only
> within SFTPGo and not in derivative works. You must develop and
> maintain your own UI components or adapt the WebAdmin/WebClient
> included in SFTPGo up to version 2.5.6.

@JohnRTitor noted that building from source might not be derivative
work, therefore marking as `unfreeRedistributable`.

[1]: https://sftpgo.com/compliance.html

Co-authored-by: Masum Reza <[e-mail]>

pkgs/servers/sftpgo/default.nix

···
       local filesystem, encrypted local filesystem, S3 (compatible) Object Storage,
       Google Cloud Storage, Azure Blob Storage, SFTP.
     '';
-    license = licenses.agpl3Only;
+    license = with licenses; [ agpl3Only unfreeRedistributable ]; # Software is AGPLv3, web UI is unfree
     maintainers = with maintainers; [ thenonameguy ];
     mainProgram = "sftpgo";
   };