[Term Entry] Java retainAll()

[devesh-2002]

Description

Added a retainAll.md file. Added content related to retainAll() in Java.

Issue Solved

Closes Issue #4346

Type of Change

• Adding a new entry

Checklist

• [x] All writings are my own.
• [x] My entry follows the Codecademy Docs style guide.
• [x] My changes generate no new warnings.
• [x] I have performed a self-review of my own writing and code.
• [x] I have checked my entry and corrected any misspellings.
• [x] I have made corresponding changes to the documentation if needed.
• [x] I have confirmed my changes are not being pushed from my forked main branch.
• [x] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• [x] I have linked any issues that are relevant to this PR in the Issues Solved section.

[CLAassistant]

All committers have signed the CLA.

[devesh-2002]

I have made the changes as requested, please do check.

[devesh-2002]

Any updates regarding PR @avdhoottt?

[devesh-2002]

Any update regarding review-2? @avdhoottt @ishg-153

[devesh-2002]

@ishg-153 I have made the changes

[github-actions[bot]]

👋 @devesh-2002 You have contributed to Codecademy Docs, and we would like to know more about you and your experience. Please take a minute to fill out this four question survey to help us better understand Docs contributions and how we can improve the experience for you and our learners. Thank you for your help!

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