Fixed redundant boolean logic

[espinosa-dev]

This PR improves the toUpperCase method in strings/Upper.java by:

1. Fixing a typo in the exception message ("connot" -> "cannot").
2. Removing the redundant check Character.isLetter(currentChar). The method Character.isLowerCase(currentChar) implicitly checks if the character is a letter, making the explicit letter check unnecessary.

• 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

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.61%. Comparing base (bd7f269) to head (cc721a3).

Files with missing lines	Patch %	Lines
src/main/java/com/thealgorithms/strings/Upper.java	85.71%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7151      +/-   ##
============================================
- Coverage     78.62%   78.61%   -0.01%     
+ Complexity     6824     6822       -2     
============================================
  Files           765      765              
  Lines         22599    22600       +1     
  Branches       4443     4443              
============================================
- Hits          17768    17767       -1     
- Misses         4117     4118       +1     
- Partials        714      715       +1     

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