Refactor `AtbashCipher`, add `ParameterizedTest`

[Hardvan]

• [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

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 70.78%. Comparing base (20239f2) to head (4be1a7e).

Files with missing lines	Patch %	Lines
...n/java/com/thealgorithms/ciphers/AtbashCipher.java	60.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5808      +/-   ##
============================================
- Coverage     70.79%   70.78%   -0.02%     
+ Complexity     4862     4860       -2     
============================================
  Files           650      650              
  Lines         17533    17533              
  Branches       3379     3379              
============================================
- Hits          12413    12410       -3     
- Misses         4656     4659       +3     
  Partials        464      464              

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