Replace the deprecated exec_program call with execute_process

[azeey]

🦟 Bug fix

Summary

exec_program is deprecated and causes the following warning

  Policy CMP0153 is not set: The exec_program command should not be called.
  Run "cmake --help-policy CMP0153" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  Use execute_process() instead.

This replaces it with execute_process as suggested by the warning.

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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[azeey]

Note, this file is a copy of https://github.com/gazebosim/gz-cmake/blob/ddd38ff196640024d6e054ff59cf5fea1ef01d73/cmake/cmake_uninstall.cmake.in