CMake: require CMake 3.5 or later

- Konqueror requires KDE Frameworks 5.71 or later,
- Frameworks 5.71 required CMake 3.5 or later,
- so match the versions.

This would be nicer to bump to 3.16, which is what current KF5
versions require; right now find_package(KF5) prints a warning
(just a warning) when the consumer requires an older CMake version.