Xcode: reject legacy buildsystem for Xcode 14

With Xcode 14 support for the Legacy Build System has been removed
and the BuildSystemType in the WorkspaceSettings is ignored.
If CMake still generates projects targeted to the Lecacy Build
System the build preparation phase will likely fail because Xcode
for example misses any declared outputs from script invocations.

This is a hard to debug problem and CMake should reject the invalid
configuration instead.