fix: remove sm_70 and sm_75 from default GPU architecture list

[egull]

Summary

• sm_70 (Volta) and sm_75 (Turing) were dropped in CUDA 13. Having them as unconditional defaults in CudaArchitectures.cmake causes a hard build failure (nvcc fatal: Unsupported gpu architecture 'compute_70') on any system with CUDA 13+.
• Removed both from the default list. The minimum default architecture is now sm_80 (Ampere, CUDA ≥ 11.0).
• Users who require Volta/Turing support can still pass -DGPU_ARCHS="70;75" explicitly when using a CUDA 12 or older toolchain.

Test plan

• Verify default build succeeds on CUDA 13+ without -DGPU_ARCHS
• Verify -DGPU_ARCHS="70;75" still works on CUDA 12

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.70%. Comparing base (1c367bb) to head (360b725).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #17   +/-   ##
=======================================
  Coverage   96.70%   96.70%           
=======================================
  Files          13       13           
  Lines         850      850           
=======================================
  Hits          822      822           
  Misses         28       28           

Flag	Coverage Δ
unittests	96.70% <ø> (ø)

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