[lldb][windows] add a warning if CMake's version is inferior to 3.30 (#164346)
https://github.com/llvm/llvm-project/pull/140443 makes use of the CMake
variable `Python3_EXECUTABLE_DEBUG`, which was introduced in CMake
version 3.30. On systems with an inferior version of cmake, the lit
tests will try to run with an empty `config.python_executable`.
This PR adds a warning and falls back to using `Python3_EXECUTABLE` if
the CMake version is less than `3.30`.