In Qt Creator just creating a blank project, under the Projects tab, in initial CMake settings, the ANDROID_STL is set to cxx_shared. Newer cmake modules in Android NDK don't accept cxx_shared, but accept c++_shared. I used that, and CMake no longer complains.
The source of the issue is android.toolchain.cmake, line 377, residing in ndk/21.1.6352462/build/cmake/:
Please make c++_shared the default initial variable for ANDROID_STL under the build settings for Android kits.