This adds an auto-update for gradle by using https://github.com/gradle-update/update-gradle-wrapper-action.

I quote from the action's manual for the Y:

Why should I use this action?

Gradle Wrapper is the recommended way to setup Gradle in your project. The Wrapper is a script that downloads and invokes a declared Gradle version. By checking the Wrapper into version control you make the build process more standardised, easy and reliable.

Maintaining dependencies up-to-date is regarded as a good practice, and the build system files should be no exception. Unfortunately, often times developers add the Wrapper to the project repository once and just forget about it.

The Update Gradle Wrapper Action aims to help keeping Gradle projects on GitHub polished to high standards and to strengthen the software supply chain in the Gradle ecosystem.
Hygiene

Gradle is under heavy development, with new releases available every few weeks. Projects that stick to old Wrapper versions can't benefit from the tons of features and bug fixes being rolled out.

Updating the build system only once or twice per year might become a tedious task. You will need to walk through longer changelogs and handle any breaking change. Updating frequently helps do less work, take advantage of new features earlier and safely drop deprecated functionality.

This action runs automatically at the declared schedule and creates Pull Requests with detailed information about the version update.

• 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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.