Transfer class: code improvements

[blackcoder87]

Description

• Code style improvements.
• Remove curlOpt array as the stored cURL options where never used after saving them in that array. The way the options where stored wasn't useful either as a cURL option is "tied" to a specific transfer (transferUrl). This shouldn't affect the functionality as the important function call for cURL is curl_setopt().
• Remove getVersionsList() as it was only used in newVersionFound() of the transfer class itself and just one time. A second call was unnecessary.
• Don't set zipFile in newVersionFound(). This caused an error (under PHP 8.2) and looks like it wasn't needed.
• Verify file right before installing.
• BS5 vorbereitung #962

Tested the changes by using this new Transfer.php in an install of Ilch 2.1.60 to update it to Ilch 2.2.2 and updating all modules. Tested installing a module and a layout as well.

There is still more work to be done, but I am planning to commit these changes at first.

Type of change

• Bug fix (non-breaking change which fixes an issue)

This PR has been tested in the following browsers:

• Chrome
• Firefox
• Opera
• Edge