refactor: removed main function, improved docstring, written JUnit tests for `KrishnamurthyNumber`.

[Tanmay-Singh3004]

• [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.
• [ ] 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

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.97%. Comparing base (efb16c1) to head (65faa27).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5881      +/-   ##
============================================
+ Coverage     66.86%   66.97%   +0.10%     
- Complexity     4516     4521       +5     
============================================
  Files           613      613              
  Lines         16996    16989       -7     
  Branches       3282     3281       -1     
============================================
+ Hits          11364    11378      +14     
+ Misses         5183     5163      -20     
+ Partials        449      448       -1     

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