upgrade to flysystem v3

[marufmax]

Summary

Currently this middleware supporting only flysystem v1 was released 8 years ago(19th of January in 2015). This PR added support for newer version flysystem v3

[Kevinrob]

Thank you for this PR!

The CI fail because we support PHP 7.
league/flysystem[3.16.0, ..., 3.17.0] require php ^8.0.2 -> your php version (7.2.34) does not satisfy that requirement.
But PHP 7 is end-of-life. So maybe we can drop it? What do you think?

[marufmax]

I think dropping support for PHP 7 would be a breaking change to this plugin. Flysystem v2 supports PHP 7.2. If you agree, I can create a separate PR to release a new version with Flysystem v2. Then we can release another version with PHP 8 and Flysystem v3.

[Kevinrob]

I think dropping support for PHP 7 would be a breaking change to this plugin. Flysystem v2 supports PHP 7.2. If you agree, I can create a separate PR to release a new version with Flysystem v2. Then we can release another version with PHP 8 and Flysystem v3.

The support of PHP <8 is now dropped on the master branch.
@marufmax Can you update your PR?