Replace join_paths with cmake_path(APPEND)

[scpeters]

🦟 Bug fix

Fixes a cleanup comment

Summary

There is a TODO comment suggesting that we use cmake_path(APPEND) when cmake 3.20 is required. Since Ionic uses cmake 3.22.1, we can make this change now.

Contents of gz-cmake4.pc with this branch:

prefix=${pcfiledir}/../../
includedir=${prefix}/include/gz/cmake4

Name: Gazebo cmake
Description: Build system package for the Gazebo libraries
Version: 4.2.0
Requires:
Requires.private:
Libs:
Libs.private:
CFlags: -I${includedir} 

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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