[ci] upgrade to GoogleTest v1.14.0 (fixes #5976)

[jameslamb]

Fixes #5976

Updates to the latest version of GoogleTest, v1.14.0 (https://github.com/google/googletest/releases), to get recent miscellaneous bugfixes and improvements and to keep up with versions that LightGBM contributors and uses are more likely to have.

Other changes:

• removes C++ standard from flags in CMakeLists.txt, in favor of setting CMake variable CMAKE_CXX_STANDARD
• sets CMAKE_CXX_STANDARD_REQUIRED to ON, to ensure a loud error is raised if using a compiler that doesn't support the requested standard (instead of silently falling back to an earlier version, which could lead to cryptic errors or undefined behavior)
• uses C++14 standard when running the C++ tests with GoogleTest

Why only go to C++14?

C++14 is the minimum version supported by newer versions of GoogleTest.

I tried to go all the way to C++20 in this PR, but encountered some compilation errors like the following:

* LightGBM/include/LightGBM/utils/common.h:1203:35:   in 'constexpr' expansion of 'fmt::v8::basic_format_string<char, const unsigned int&>(format)'
* LightGBM/include/LightGBM/utils/common.h:1203:35: error: 'format' is not a constant expression

Which might be related to fmtlib/fmt#2775 or fmtlib/fmt#2898.

I'm proposing separating the work of adding C++20 out to #6033.

Adding C++14 specifically also improves the project's CI coverage of C++ standards:

• C++11 = most tests (Python-package, CMake-based R-package, SWIG, CUDA): C++11
• C++14 = the cpp tests
• C++17 = CRAN-style R-package tests

References

The changes in CMakeLists.txt are supported by the following:

• https://cliutils.gitlab.io/modern-cmake/chapters/features/cpp11.html
• https://crascit.com/2015/03/28/enabling-cxx11-in-cmake/

How I tested this

git clean -d -f -X
rm -rf ./build
mkdir build
cd ./build
cmake -DBUILD_CPP_TEST=ON -DUSE_OPENMP=OFF ..

VERBOSE=1 \
make -j2 testlightgbm

../testlightgbm

Observed that the expected --std flags were passed to the compiler (e.g.

[jameslamb]

Ok good! Just updating the version without changing the C++ standard, I see exactly the same errors in CI as reported in #5976.

/__w/1/s/build/_deps/googletest-src/googletest/include/gtest/internal/gtest-port.h:270:2: error: C++ versions less than C++14 are not supported.
#error C++ versions less than C++14 are not supported.

https://dev.azure.com/lightgbm-ci/lightgbm-ci/_build/results?buildId=14929&view=logs&j=c1aa5445-36f0-58f5-2c69-68691d629457&t=c07c8f5b-ba24-5225-6ddc-80219dbcdc35

[jameslamb]

@guolinke @shiyu1994 could I please get a review on this one?

[jameslamb]

thank you!

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