Fix build with poppler 21.8.0

parseDateString() now takes "const GooString *" as the date argument in
order to support unicode date strings. Adjust parsePdfDateTime() to use
the same string type (instead of "const char *") when compiling against
poppler 21.8.0.

https://gitlab.freedesktop.org/poppler/poppler/-/merge_requests/880
2 jobs for release/21.08 in 10 minutes and 3 seconds (queued for 4 seconds)