update gtest version to 1.12.1
FYI: Currently, following configuration files are referencing gtest `release-1.12.1`.

- tensorflow/workspace2.bzl
- tensorflow/tools/pip_package/xla_build/pip_test/CMakeLists.txt
When I tried to build tf-lite with kernel test via cmake, I encounted build error.

This patch fixes the gtest version to resolve this problem.