Use umi_count instead of duplicate_count in compliance with AIRR v1.4

[grst]

• Remove index_chains_legacy because the numba implemenation seems to work fine
• Use 'umi_count' instead of 'duplicate_count'

Closes #478

• CHANGELOG.md updated
• Tests added (For bug fixes or new features)
• Tutorial updated (if necessary)

[grst]

@zktuong, @amoschoomy, FYI in this PR I update scirpy to use umi_count instead of duplicate_count which is recommended since AIRR v1.4 (see release notes). Maybe you want to do the same thing in Dandelion?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (84f390f) 80.50% compared to head (e85f437) 81.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
+ Coverage   80.50%   81.04%   +0.53%     
==========================================
  Files          49       49              
  Lines        4012     3972      -40     
==========================================
- Hits         3230     3219      -11     
+ Misses        782      753      -29     

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