fix: update CONTRIBUTING.md to reference Discord instead of Gitter

[prasanth-30011]

Updated contact information for questions about the repository.

• [ ] I have read CONTRIBUTING.md.
• [ ] This pull request is all my own work -- I have not plagiarized it.
• [ ] All filenames are in PascalCase.
• [ ] All functions and variable names follow Java naming conventions.
• [ ] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [ ] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[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 (0815490).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7128      +/-   ##
============================================
- 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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