[clang-tidy] Update tests to include C++23 and C++26

This commit changes the `c++xx-or-later` definitions to also include
C++23 and the upcoming C++26.
`readability/container-contains.cpp` to also test newer C++ versions.

Also, this commit adjusts a couple of test cases slightly:
* `container-contains.cpp` now also tests newer C++ versions.
  Restricting it to C++20 was an oversight of mine when originally
  writing this check.
* `unconventional-assign-operator.cpp`: The `return rhs` raised
  a "non-const lvalue reference to type 'BadReturnStatement' cannot
  bind to a temporary" error in C++23. The issue is circumenvented
  by writing `return *&rhs`.
* `const-correctness-values.cpp` was also running into the same error
  in C++23. The troublesome test cases were moved to a separate file.

Differential Revision: https://reviews.llvm.org/D157246