chore: Update DEVELOPMENT.md with LLVM 21 setup instructions

[aborgna-q]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.78%. Comparing base (61aba25) to head (b58e035).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2935   +/-   ##
=======================================
  Coverage   83.78%   83.78%           
=======================================
  Files         267      267           
  Lines       52975    52975           
  Branches    46914    46914           
=======================================
  Hits        44384    44384           
  Misses       6320     6320           
  Partials     2271     2271           

Flag	Coverage Δ
python	88.74% <ø> (ø)

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