build: Update C++ standard from c++17 to c++20 (#129)

[mc-nv]

Cherry-pick of the C++17->C++20 standard bump (TRI-1877) onto r26.09.

Upstream PyTorch raised ATen's minimum required C++ standard from 17
to 20 (pytorch/pytorch#178150), which broke the pytorch_backend build.
Align this repo's TRITON_MIN_CXX_STANDARD/CMAKE_CXX_STANDARD default
with that floor for consistency across the stack.

TRI-1877

(cherry picked from commit af40652)

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge because it consistently updates every repository CMake entry point to the required C++20 baseline.

Summary

• Updates the root backend utility and optional CUDA kernel build.
• Updates the minimal, recommended, and BLS example backends.
• Updates the single- and volume-batching strategy examples.
• Refreshes copyright years to 2026.

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