Set KDE_COMPILERSETTINGS_LEVEL

Since the KF5 version requirement was incremented to 5.86 the build
started to fail (at least with KF 5.87.0) due to deprecations.

CMakeLists.txt

@@ -7,6 +7,7 @@ set(CMAKE_C_STANDARD 99)
 
 set(QT_MIN_VERSION "5.15.0")
 set(KF5_MIN_VERSION "5.86")
+set(KDE_COMPILERSETTINGS_LEVEL "5.82")
 
 find_package(ECM ${KF5_MIN_VERSION} REQUIRED NO_MODULE)