Follow-up to #9797

Two optimizations:

1. Optimize the compiler: The Java compiler is fast. But if you’re compiling hundreds of Java classes, even a short compilation time adds up.

2. Enable the build cache: The build cache is a Gradle optimization that stores task outputs for specific input. When you later run that same task with the same input, Gradle retrieves the output from the build cache instead of running the task again.

I kept a third optimization for future work.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.