bump cmake to 2.8.12

[ijnek]

🦟 Bug fix

Bug

Deprecation warning when building with CMake >3.19.1:

--- stderr: ignition-tools
  CMake Deprecation Warning at CMakeLists.txt:2 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.
---

Summary

Prevents a deprecation warning that arises since CMake3.19.1: Similar PR has been raised and merged in https://github.com/facebook/zstd/pull/2399.

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] 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.