Follow-up to #10227

Remove JSpecify.

Although JSpecify is cool and annotations can be made, the concrete usage should be more "useful". A follow-up PR can add a real example, code howto, and an ADR.

Link to the stackoverflow question for null annotations: https://stackoverflow.com/q/4963300/873282

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.