When building with Qt6, disable as much deprecated code as possible

And ensure deprecation warnings for bot Qt and KF versions higher than the
specified ones are shown when building.

CMakeLists.txt

@@ -45,6 +45,17 @@ include(CheckIncludeFiles)
 # Allows passing e.g. -DECM_ENABLE_SANITIZERS='address;undefined' to cmake.
 include(ECMEnableSanitizers)
 
+if(QT_MAJOR_VERSION EQUAL "6")
+    # ECMDeprecationSettings is in ECM since 5.91, this assumes if you're building
+    # with Qt6, you have latest ECM anyway
+    include(ECMDeprecationSettings)
+    ecm_set_disabled_deprecation_versions(
+        SHOW_DEPRECATIONS
+        QT 5.15.2
+        KF 5.93.0
+    )
+endif()
+
 ecm_setup_version(${RELEASE_SERVICE_VERSION} VARIABLE_PREFIX KONSOLEPRIVATE
     SOVERSION 1
 )