Simple processor and Simple Log processor use a non-standard feature deprecated in C++20

[maxgolov]

These two pieces of code:

opentelemetry-cpp/sdk/include/opentelemetry/sdk/trace/simple_processor.h

Line 75 in 68e510c

std::atomic_flag shutdown_latch_{ATOMIC_FLAG_INIT};

opentelemetry-cpp/sdk/include/opentelemetry/sdk/logs/simple_log_processor.h

Line 62 in 106c00a

std::atomic_flag shutdown_latch_{ATOMIC_FLAG_INIT};

Should be rewritten as follows:

• assignment for anything that is below C++20, as shown here: https://en.cppreference.com/w/cpp/atomic/ATOMIC_FLAG_INIT

std::atomic_flag static_flag = ATOMIC_FLAG_INIT; // static initialization,

• and empty constructor for C++20, since This macro is no longer needed and deprecated, since default constructor of std::atomic_flag initializes it to clear state.

I hit this while rebasing my PR for Standard Library support and testing it with C++20. The code won't compile as written in Visual Studio 2019 with C++20 enabled.

[maxgolov]

I'll fix it.

[maxgolov]

I added the fix in #374