build: Update C++ standard from c++17 to c++20 (#24)

[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 094675c)

[greptile-apps]

Confidence Score: 4/5

The PR should not merge until existing configured build trees are prevented from silently retaining the obsolete C++17 setting.

Findings

1.  Cached C++17 Remains Active ▶

Summary

• Aligns compilation with dependencies that now require C++20.
• Existing CMake build trees can retain the previous cached C++17 value, leaving the intended floor unenforced.

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

[greptile-apps]

Cached C++17 Remains Active

When an existing build tree is reconfigured after this update, CMake preserves its cached TRITON_MIN_CXX_STANDARD=17 because this assignment only changes the default. The target then continues requesting cxx_std_17, so builds that include C++20-only dependency headers can still fail despite this standard bump. Please reject values below 20 or migrate the stale cached default.