style: enable MultipleVariableDeclarations in checkstyle

[vaibhav9t1]

Related to #5119
Enabled the MultipleVariableDeclaration in checkstyle

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All functions and variable names follow Java naming conventions.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 48.38710% with 80 lines in your changes are missing coverage. Please review.

Project coverage is 38.60%. Comparing base (44ce6e7) to head (f1148a1).

Files	Patch %	Lines
...healgorithms/datastructures/trees/RedBlackBST.java	0.00%	5 Missing ⚠️
...main/java/com/thealgorithms/maths/KeithNumber.java	0.00%	5 Missing ⚠️
...ain/java/com/thealgorithms/ciphers/HillCipher.java	0.00%	4 Missing ⚠️
...com/thealgorithms/conversions/DecimalToBinary.java	0.00%	4 Missing ⚠️
...ealgorithms/datastructures/graphs/BellmanFord.java	0.00%	4 Missing ⚠️
...va/com/thealgorithms/datastructures/trees/LCA.java	0.00%	4 Missing ⚠️
...a/com/thealgorithms/maths/DeterminantOfMatrix.java	0.00%	4 Missing ⚠️
...n/java/com/thealgorithms/others/GuassLegendre.java	0.00%	4 Missing ⚠️
...n/java/com/thealgorithms/conversions/AnytoAny.java	0.00%	3 Missing ⚠️
...a/com/thealgorithms/maths/NonRepeatingElement.java	0.00%	3 Missing ⚠️
... and 21 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5175      +/-   ##
============================================
+ Coverage     38.56%   38.60%   +0.03%     
+ Complexity     2378     2377       -1     
============================================
  Files           516      516              
  Lines         15313    15391      +78     
  Branches       2957     2957              
============================================
+ Hits           5906     5941      +35     
- Misses         9121     9163      +42     
- Partials        286      287       +1     

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