Update composer.json with `league/flysystem-sftp-v3`

[forsplay]

Installing banago/phploy (v4.9.1): Extracting archive 2 package suggestions were added by new dependencies, use `composer suggest` to see details. 
Package league/flysystem-sftp is abandoned, you should avoid using it. 
Use league/flysystem-sftp-v3 instead. 
Generating autoload files

Updating this dependency.