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

[koppor]

🤖 Generated with Claude Code

Merging main into this branch conflicts in 1 file. I did not push a resolution.

.github/workflows/deployment-jdk-ea.yml is a modify/delete: main deleted it in the multi-project split (JabRef#12990, 518f61f4fc) and has no EA/Valhalla workflow left at all. Keeping this branch's copy would resurrect a workflow written against the old single-module Gradle build.

Whether the EA deployment workflow should come back, rewritten for the module layout, is a human call rather than a conflict resolution.

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[koppor]

🤖 Generated with Claude Code

Merged main into ea-valhalla to resolve the merge conflict.

Conflicting file: .github/workflows/deployment-jdk-ea.yml — a modify/delete conflict. main deleted the workflow in the multi-project build change (JabRef#12990); this PR's only content is its edit to that same file, so I kept this branch's version rather than taking the deletion, which would have emptied the PR. Note the restored workflow still references the pre-split build.gradle layout, so it likely needs a follow-up to match the multi-project structure.

No build was run locally; this PR's CI is the compile check.

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Generated by Claude Code

[koppor]

🤖 Generated with Claude Code

Merged main into ea-valhalla to resolve the merge conflict.

• Conflicting file: .github/workflows/update-experimental.yml — an add/add conflict. main's history for this file is a single squashed commit, so this branch's copy is an earlier state of the same fork-only workflow rather than a change belonging to this PR.
• Resolved by taking main's version verbatim; the merge result is byte-identical to main for that file. Nothing else in this PR touches it.

CI on this PR is the compile check.

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Generated by Claude Code