Revert "Update qt5 version requirement to 5.15.0"

This reverts commit 74992528.

CMakeLists.txt

@@ -6,7 +6,7 @@ set(PROJECT_VERSION "5.19.90")
 set(PROJECT_VERSION_MAJOR 5)
 
 # check with non-Plasma consumers (e.g. KDevelop) before bumping these versions to make sure the KDE CI does not break
-set(QT_MIN_VERSION "5.15.0")
+set(QT_MIN_VERSION "5.14.0")
 set(KF5_MIN_VERSION "5.74")
 
 find_package(ECM ${KF5_MIN_VERSION} REQUIRED NO_MODULE)