Fix C++20 warning for deprecated enum operation

[Repiteo]

Small cleanup to circumvent repos targeting C++20 and above from having to suppress warnings over deprecated enum operations. The offending area was X::num_rows + Y::num_rows in the template declaration of matrix_combine_vertically, resulting in these warnings:

• GCC: deprecated-enum-enum-conversion
• Clang: deprecated-anon-enum-enum-conversion
• MSVC: 5054 (operator '*': deprecated between enumerations of different types)

This was addressed by converting the offending file's anonymous enums to static constexpr variables, which is the recommended way to declare scoped constants in C++17 and above.