Plan move to C++17

[marcalff]

According to the Abseil doc:

• https://github.com/abseil/abseil-cpp?tab=readme-ov-file#support

support for C++14 will be revisited on 2024-12-15:

• https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

Per policies:

• https://opensource.google/documentation/policies/cplusplus-support

C++ 14 will be dropped for C++17:

We will drop support for our oldest supported C++ standard when one of the following happens:

    All supported compilers* default to a newer version
    When 10 years pass since the standard's release date
    Example: In 2023, if all supported compilers use C++17 as their default, then we can drop support for C++14. Otherwise, we'll need to wait until 2024, which would be 10 years after C++14 was released.

When abseil no longer supports C++14 and requires C++17, so will gtest, gmock, grpc, overall forcing opentelemetry-cpp to drop C++14 as well.

We need to plan for this

[sjinks]

The code does use C++20 extensions (specifically, sdk/include/opentelemetry/sdk/common/global_log_handler.h does). You can confirm this by building with -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_CXX_FLAGS="-Wc++20-extensions -Wc++20-compat-pedantic"

/home/volodymyr/github/opentelemetry-cpp/sdk/src/logs/logger_provider.cc:35:3: warning: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Wc++20-extensions]
   35 |   OTEL_INTERNAL_LOG_DEBUG("[LoggerProvider] LoggerProvider created.");
      |   ^
/home/volodymyr/github/opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/global_log_handler.h:207:40: note: expanded from macro 'OTEL_INTERNAL_LOG_DEBUG'
  207 | #  define OTEL_INTERNAL_LOG_DEBUG(...) OTEL_INTERNAL_LOG_DEBUG_MACRO(__VA_ARGS__)(__VA_ARGS__)
      |                                        ^
/home/volodymyr/github/opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/global_log_handler.h:206:65: note: expanded from macro 'OTEL_INTERNAL_LOG_DEBUG_MACRO'
  206 |                                   OTEL_INTERNAL_LOG_DEBUG_1_ARGS)
      |                                                                 ^
/home/volodymyr/github/opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/global_log_handler.h:165:9: note: macro 'OTEL_INTERNAL_LOG_GET_3RD_ARG' defined here
  165 | #define OTEL_INTERNAL_LOG_GET_3RD_ARG(arg1, arg2, arg3, ...) arg3

/home/volodymyr/github/opentelemetry-cpp/sdk/src/trace/tracer_provider.cc:90:5: warning: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Wc++20-extensions]
   90 |     OTEL_INTERNAL_LOG_ERROR("[TracerProvider::GetTracer] Library name is null.");
      |     ^
/home/volodymyr/github/opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/global_log_handler.h:177:40: note: expanded from macro 'OTEL_INTERNAL_LOG_ERROR'
  177 | #  define OTEL_INTERNAL_LOG_ERROR(...) OTEL_INTERNAL_LOG_ERROR_MACRO(__VA_ARGS__)(__VA_ARGS__)
      |                                        ^
/home/volodymyr/github/opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/global_log_handler.h:176:65: note: expanded from macro 'OTEL_INTERNAL_LOG_ERROR_MACRO'
  176 |                                   OTEL_INTERNAL_LOG_ERROR_1_ARGS)
      |                                                                 ^
/home/volodymyr/github/opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/global_log_handler.h:165:9: note: macro 'OTEL_INTERNAL_LOG_GET_3RD_ARG' defined here
  165 | #define OTEL_INTERNAL_LOG_GET_3RD_ARG(arg1, arg2, arg3, ...) arg3
      |         ^
...

/home/volodymyr/github/opentelemetry-cpp/sdk/test/metrics/base2_exponential_histogram_indexer_test.cc:149:27: warning: hexadecimal floating literals are a C++17 feature [-Wc++17-extensions]
  149 |   EXPECT_EQ(compute_index(0x1.FFFFFFFFFFFFFp1023), 63);

(I didn't build the entire library; there could be more compatibility issues)