[various] Upgrade googletest to v1.15.2 (#11586)

Running e.g. pigeon tests (`packages\pigeon\tool\test.dart`) with Visual Studio 2026 gives the following error:

```
##############################
# Generating platform_test/ output [start time 22:58:25]
Generation complete!
##############################
# Running windows_unittests [start time 22:58:29]
Building Windows application...                                 
CMake Error at D:/Tmp/Coding/git/flutter/packages/packages/pigeon/platform_tests/test_plugin/example/build/windows/x64/_deps/googletest-src/CMakeLists.txt:4 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

Building Windows application...                                     6,9s
Unable to generate build files
# Failed, exit code: 1

Process finished with exit code 1
```

So upgrading the google tools to v1.15.2 resolves that issue as since [1.12.1](https://github.com/google/googletest/releases/tag/release-1.12.1) the minimum required CMake version is 3.5.

Towards https://github.com/flutter/flutter/issues/185597

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