neorados/test: use [[maybe_unused]] instead of GTEST_ATTRIBUTE_UNUSED_

neorados/test: use [[maybe_unused]] instead of GTEST_ATTRIBUTE_UNUSED_

This change:

- Aligns with GoogleTest's own evolution, which now uses
  `[[maybe_unused]]` internally (via
  `GTEST_INTERNAL_ATTRIBUTE_MAYBE_UNUSED`) as of
  https://github.com/google/googletest/commit/5a37b517
- Leverages C++20 features already available in Ceph
- Removes dependency on GoogleTest internal macros
- Enables updating the GoogleTest submodule to silence CMake warnings

Using the standard C++ attribute provides better compatibility and
future-proofing than relying on GoogleTest's internal implementation
details.

This change also allows us to bump up googletest submodule to silence a
CMake warning.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>