gz-cmake-extras.cmake.in uses cmake_path which was introduced in cmake 3.20 but minimum required version is 3.10.2

[azeey]

Environment

• OS Version: Ubuntu 22.04
• Source or binary build? gz-msgs10

Description

• Expected behavior: gz-msgs to use CMake features available in 3.10.2 since that's the minimum declared in the top CMakeLists.txt file.
• Actual behavior: gz-cmake-extras.cmake.in uses cmake_path which was introduced in cmake 3.20. This might cause build problems for some users (e.g., https://robotics.stackexchange.com/a/107914/31574)

[mjcarroll]

Yes, that is certainly incorrect. I didn't realize cmake_path was that new. I can update the logic to avoid it.