Fix IndexTransformIter to be compatible with C++23

[StephanTLavavej]

Fixes #9127.

In the STL's conventions, an iterator's value_type should always be "plain" - no cv-qualifiers and no references. Although this convention dates back to C++98, code could sometimes violate it and still compile, depending on the exact operations used by any instantiated templates. However, now that C++ has evolved to perform concept checking, stricter enforcement is happening within STL algorithms. XGBoost fails to compile with MSVC's /std:c++latest mode because its iterator IndexTransformIter has a value_type that doesn't follow the proper conventions.

This PR is my attempted fix, which appears to resolve the compilation issues, and "seems reasonable" to me - it changes the value_type to be plain by using remove_cvref_t (but written in the older style to avoid depending on the C++20 convenience type trait), while reference is the type that's being returned by the Fn invocation.

Note that the actual return types of operator* and operator[] are being preserved (they continue to return the result_of_t), it's just the mentioned typedef that's changing.

Because I work on the C++ Standard Library all day and haven't actually used XGBoost or run its tests, this PR could have subtle implications elsewhere. I hope that it's correct or at least a good start.

[trivialfis]

Thank you for the fix!