Update tuple to C++20

[tretre91]

This PR updates the tuple to C++20, what's been done:

• remove the CEXA_HAS_CXX20 macro
• unguard test code inside #ifdef TEST_STD_VER >= 20
• get rid of the helper classes for conditionally deleting the assignment operators and use requires instead
• replace some sfinae with a dummy template parameter with concepts
• replace some fallback type traits with the C++20 equivalent from the stdlib
• silence the host function called from host device function warnings when using nvcc
• fix the clang tidy warnings in the implementation (warnings in the tests are not handled by this PR)