This string is not the banner. It's SSH software version string. Those strings are supposed to be used by SSH implementations to decide whether to apply various workarounds - see RFC 4253:
The 'softwareversion' string is primarily used to trigger compatibility extensions and to indicate the capabilities of an implementation.
This means that changing the string could trigger strange interoperability issues that would not occur otherwise. And for this reason, we decided not to make it customizable. This said, if you really need to change that string, please get in touch with us at email@example.com to discuss this.
FileServer.Settings.Banner property is used to specify a banner message to be sent to the client during authentication.)