Switch to C++20 as a minimum requirement

[tpadioleau]

The PR bumps the minimum C++ standard from C++17 to C++20 (targeting CUDA 12.2 and ROCm 6.2 as the minimum supported backends). It includes three main changes:

• Setting C++20 as the minimum requirement across the build system
• Updating the CI and the oldest Docker image accordingly
• Adding NVCC workarounds for compiler issues
• Fixing modernize-use-constraints clang-tidy warnings by introducing proper C++20 concepts in the namespace ddc::concepts
• Disabling modernize-use-integer-sign-comparison

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.89%. Comparing base (f5b37e7) to head (d57ae5c).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1086      +/-   ##
==========================================
- Coverage   94.91%   94.89%   -0.02%     
==========================================
  Files          64       64              
  Lines        2849     2840       -9     
  Branches      892      892              
==========================================
- Hits         2704     2695       -9     
  Misses         95       95              
  Partials       50       50              

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