Add Java Streams example with Real world scenario

[hirushi1999]

Hi @dimpeshpanwar,
I would like to contribute a Java example for Hacktoberfest 2025

The code demonstrates Java Streams and Collectors with an Employee list, including:

• Counting employees by age groups.
• Finding the highest salary per department among active employees.
• Listing all active employees.

Please let me know if I can proceed with this contribution.

[dimpeshpanwar]

Yes. You can

[hirushi1999]

Thanks a lot, @dimpeshpanwar