Fix/implement C++2020 compilation, tests, and CI

[dmitrykobets-msft]

The C++2020 features (std::span support) were not enabled in the CI. As a result, the code had compilation issues that were never caught. Furthermore, the compiler versions in the README did not match up to the compilers that were actually being tested. For instance, the minimum GCC version advertised is 8, but we are testing 7.5.0 instead. The precise versions we should be testing will be re-visited in a later PR. But for now, this PR simply updates the README to be accurate to what is currently being tested.

Compiler	Advertised (before this PR)	Actually Tested
XCode	11.4 & 10.3	12.4 & 11.3
GCC	9 & 8	9.3.0 & 7.5.0
Clang	11 & 10	11.0.0 & 9.0.0
Visual Studio with MSVC	VS2017 (15.9) & VS2019 (16.4)	VS2017 (15.9.21) & VS2019 (16.11.2)
Visual Studio with LLVM	VS2017 (Clang 9) & VS2019 (Clang 10)	none	none

Previously, the guard for the span tests was __cplusplus >= 202002L. Unfortunately, this does not work for all compilers. For instance, in GCC 10, running with -std=c++20 will fail the above condition (__cplusplus will still be at 2017), and yet __cpp_lib_span >= 202002L will be true. But for GCC 9 and GCC 8, __cplusplus will still be at 2017 and __cpp_lib_span is undefined (neither have <span> support). In all cases above, the span tests will be silently skipped. To avoid this confusing / incorrect (in the case of GCC 10) behavior, make the span tests execute unconditionally if running in CI mode with C++20 enabled. If a compiler version does not support span, the C++20 version should be explicitly removed from its CI jobs.

[SloppyJaconda]

Out of curiosity, is there a specific issue that you're looking to fix with this?

[dmitrykobets-msft]

@SloppyJaconda It appears that the C++20 feature support actually had compilation / syntax errors. This was hidden because C++20 was never part of the CI pipeline, so the tests were never run. So this PR fixes those errors and attempts to enable C++20 in the CI