Down to JDK19

[koppor]

Switch to Java 19 for gradle build

It seems that the toolset around VS Code does not support Java 20. See redhat-developer/vscode-java#3043 (comment).

I think, we do not use new features of JDK 20 yet, thus I propose the downgrade.

JabRef can still be compiled with JDK 20.

Compulsory checks

Beta Give feedback

1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. 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.