Update gradle from 8.14.2 to 9.0.0-rc-1

[koppor]

With #13401, we are ready for the next gradle generation.

I would propose to get this in to gain experiences

Gradle release notes: https://gradle.org/release-candidate/

Steps to test

1. ./gradlew :jabgui:run
2. Reimport in IntelliJ

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
• [/] Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• [/] 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.

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨