Upgrade to Slim 4 for PHP 8

[cambell-prince]

Upgrade to Slim 4 for PHP 8

• Note that this will break compatibility with older PHP versions.
• Still, older releases of Simple API will continue to work with older versions of PHP.

See this FrontAccounting forum post.

TODO:

• Redo api_login as middleware
• JSON encoding as middleware
• Refactor use of getInstance(...)

[DrCard]

Dear any update

if you started i would like to finish where you stopped

Regards