`<xstring>` is the MSVC internal implementation file for string. This makes it contingent on the MSVC STL.
`<string>` provides the same declarations, but does not expose the MSVC-specific implementation details. This implementation is guaranteed to meet the C++ standard exactly.
This change updates includes of `<xstring>` to be includes of `<string>`.
PiperOrigin-RevId: 483696244