Update GoogleTest to 1.12.x for more recent CMake support

[PhilMiller]

The existing pointer to GoogleTest 1.8.x refers to an old enough CMake version that the latest 4.0 no longer supports it. Updating to GoogleTest 1.12.x brings that up to 3.5, such that support is available.

Changes

• Change the extern/googletest submodule branch pointer

Testing

1. Built and ran locally

Notes

• Newer versions may work, too. I just didn't feel like testing and digging into potential GoogleTest API issues

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️