Require C++20 to fix the build with poppler>=24.05
poppler >= 24.05.0 started to use std::string's starts_with and ends_with [1] in a public header, but these are only available with C++20.
[1] https://gitlab.freedesktop.org/poppler/poppler/-/commit/fbb64544e5ea25ac9b1bd25b48043d074efe9cd9