Update to Valhalla

[koppor]

Prepares the usage of Valhalla.

Reason: String!. See https://bugs.openjdk.org/browse/JDK-8303099.

Won't work, because of gradle toolchain lacking support for JDK23. See https://docs.gradle.org/current/userguide/compatibility.html.

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.

[koppor]

Err, even though it should not work, there are artifacts produced at https://github.com/koppor/jabref/actions/runs/10223502907?pr=692

[koppor]

Runs:

I only don't get, why 5.12 is displayed as version - and not v5.15