Remove deprecated DART_COMMON_MAKE_SHARED_WEAK

[jslee02]

Prune the legacy smart-pointer macro deprecated in DART 6.4.

• Delete DART_COMMON_MAKE_SHARED_WEAK so code uses DART_COMMON_DECLARE_SHARED_WEAK directly.
• Update documentation and examples to reference the supported macro.
• Note the removal in the DART 7 changelog.

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Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes
• Add unit tests for new functionality
• Document new methods and classes
• Add Python bindings (dartpy) if applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.65%. Comparing base (f146767) to head (8f629d1).
⚠️ Report is 375 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2127   +/-   ##
=======================================
  Coverage   61.65%   61.65%           
=======================================
  Files         393      393           
  Lines       33195    33195           
  Branches     4095     4095           
=======================================
  Hits        20466    20466           
  Misses      12729    12729           

Flag	Coverage Δ
unittests	61.65% <ø> (ø)

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