Let embedders customize GTEST_INTERNAL_DEPRECATED().

GTEST_INTERNAL_DEPRECATED is currently used to nudge googletest users to migrate off old TEST_CASE macros to the new TEST_SUITE macros. This move is non-trivial for Chromium (see https://crbug.com/925652), and might be difficult for other big projects with many dependencies.

This CL facilitates moving off of deprecated APIs by making it possible for an embedder to define GTEST_INTERNAL_DEPRECATED() in gtest/internal/custom/gtest-port.h. Example usage:

1) #define GTEST_INTERNAL_DEPRECATED() to nothing, to disable deprecation warnings while migrating off googletest's deprecated APIs. This can be preferable to having to disable all deprecation warnings (-Wno-error=deprecated or -Wno-deprecated-declarations).

2) #define GTEST_INTERNAL_DEPRECATED() for an unsupported compiler.

PiperOrigin-RevId: 236171043