3.15.1 for PS1.6.x

[ * ] fixed issue with cancel URL in case of the 'one page' order mode #26561
[ * ] fixed issues with PayPal Plus payments #25027, #25305, #26447
[ * ] fixed issue with shortcuts after switching from PayPal Standard to PP Plus #26779
[ * ] fixed issue with duplicated orders #26342
[ * ] fixed issue with guest checkout #25514
[ * ] fixed translation in DE (PR #68) #26404
[-] removed PayPal login option (not supported by PayPal Official module anymore and will be automatically disabled once the module is updated) #26616