Improve the Clipper dependency’s qt_attribution.json file

The file located at qtlocation/src/3rdparty/clipper/qt_attribution.json has many issues:

• The current description (“Clip2Tri can be used together with Clipper for robust triangulation.”) refers to the Clip2Tri project instead of Clipper. A new description should be written to match the Clipper project.
• The copyright year should be updated to match the version used in qtlocation (6.4.0)
• The website URL should be set to http://angusj.com/delphi/clipper.php so it’s easier to find the project documentation.
• The version number (6.4.0) should be added.