Update to GoogleTest best practices

[tyler-yankee]

Our CMake examples that use GoogleTest vendor it into <example>/third_party/googletest/.

According to https://google.github.io/googletest/quickstart-cmake.html, the official recommendation is to use CMake's FetchContent to obtain it and use the GoogleTest module to set up tests. DEE should be set up to follow this, instead.

[tyler-yankee]

As a follow-up to this issue, we could consider exploring whether Renovate bot can auto-update our GoogleTest version. Given the open renovatebot/renovate#42547 and a general lack of documentation, I take it this isn't supported, but as a workaround we might be able to craft a regex with customManagers that looks something like:

"customManagers": [
  {
    "customType": "regex",
    "fileMatch": ["CMakeLists\\.txt"],
    "matchStrings": [...],
    "datasourceTemplate": "..."
  }
]