build: Update C++ standard from c++17 to c++20 (#455)

[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 26764b9)

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge with no actionable issues identified.

Summary

• Changes the cached TRITON_MIN_CXX_STANDARD default from 17 to 20.
• Consequently applies C++20 to the backend, stub, and GPU compilation paths that consume this setting.

_{Reviews (1) · Last reviewed commit: "build: Update C++ standard from c++17 to..."}