CUPS can't find the pstoraster driver on your system, which doesn't exist anymore (apparently it's called gstoraster now). However, your /etc/cups/mime.convs still points to pstoraster. This file shouldn't exist anymore, as CUPS's mime conversion files are now in /usr/share/cups/mime.
Long story short, deleting /etc/cups/mime.convs might already solve your problem. The file is not part of the system anymore and won't be reinstalled.
Thank you for reporting this bug.
CUPS can't find the pstoraster driver on your system, which doesn't exist anymore (apparently it's called gstoraster now). However, your /etc/cups/ mime.convs still points to pstoraster. This file shouldn't exist anymore, as CUPS's mime conversion files are now in /usr/share/ cups/mime.
Long story short, deleting /etc/cups/ mime.convs might already solve your problem. The file is not part of the system anymore and won't be reinstalled.
Please let me know if that fixes your bug.