Update Java and Gradle versions in android-java-gradle-migration-guid…

[KarthiDreamr]

Description: This pull request enhances the android-java-gradle-migration-guide.md file by:

• Updating it with the latest Java version (21) and its compatible Gradle version (8.5).
• Including detailed instructions on how to check the JDK version using flutter upgrade and flutter doctor --verbose.

These improvements:

• Provide a more generalized solution.
• Make the guide applicable and beneficial for a wider range of developers.
• Ensure the guide remains relevant and useful for those working on Android projects with Flutter.
• Offer clear and up-to-date information to help prevent compatibility issues and streamline the setup process.