Transfer class: code improvements (#1020)

- 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