docs: Update CONTRIBUTING.md to reference Discord instead of Gitter

[vardhan30016]

Fixes #7105

The CONTRIBUTING.md file currently references "TheAlgorithms room in GITTER" for community questions, but the repository has migrated to Discord for community communication. Updated the reference to point to the Discord community instead.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 78.50%. Comparing base (2c4bf3c) to head (6d48e6c). :warning: Report is 7 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7130      +/-   ##
============================================
- Coverage     78.51%   78.50%   -0.01%     
  Complexity     6752     6752              
============================================
  Files           759      759              
  Lines         22402    22402              
  Branches       4400     4400              
============================================
- Hits          17589    17587       -2     
- Misses         4108     4109       +1     
- Partials        705      706       +1     

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