Raise the minimum supported language version to C++ 17

[doug-walker]

Based on discussion at the 2025-03-17 TSC meeting, the decision has been made to raise the minimum supported C++ language version from C++11 to C++17. Note that the VFX Platform raised to C++17 for CY2021, so OCIO supported the older versions for an extra four years, which seems like more than enough.

The work involved is in the CMake scripts and in the GitHub Actions.