Fix build with icu >=75

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -10,7 +10,7 @@ project(
     LANGUAGES CXX
 )
 
-set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD 17)
 set(QT_MIN_VERSION 5.9.0)
 # Somewhat arbitrary chosen version number ...
 set(KF5_MIN_VERSION 5.51)
