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 facebook/zstd#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)
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[chapulina]

Thanks, that should be good on all our supported platforms.