Simplify latest version extraction from packagist json file and removes deprecation message from action output

[micheleorselli]

This PR contains two changes

1. It uses a different packagist endpoint to tech package metadata, documented here in the section "Using the Composer v2 metadata" which requires less manipulation to extract the version number.
2. Switches to a new way to define output variables in action steps

[AlessandroMinoccheri]

@micheleorselli my recommendation, is to test if it's possible the workflow with https://github.com/nektos/act
It seems fine for me the new approach

[micheleorselli]

done, tested locally with act, it seems to work fine!

[AlessandroMinoccheri]

LGTM 👍