Require C++20

I want to use std::string::starts_with
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1c52b53..95a105e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -47,7 +47,7 @@
 set(POPPLER_MICRO_VERSION "0")
 set(POPPLER_VERSION "${POPPLER_MAJOR_VERSION}.${POPPLER_MINOR_VERSION_STRING}.${POPPLER_MICRO_VERSION}")
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_C_STANDARD 11)