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

[gowtham1412-p]

Fixes #7105

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] 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 (1c6026e) to head (f40f260). :warning: Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7107      +/-   ##
============================================
- Coverage     78.50%   78.50%   -0.01%     
+ Complexity     6752     6751       -1     
============================================
  Files           759      759              
  Lines         22402    22402              
  Branches       4400     4400              
============================================
- Hits          17587    17586       -1     
  Misses         4109     4109              
- Partials        706      707       +1     

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[gowtham1412-p]

@DenizAltunkapan @yanglbme @alxkm ,,,please review it once.