Update to gradle 7.4.2 and jdk18

[calixtus]

• 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]

News of Java 18: https://www.infoworld.com/article/3630510/jdk-18-the-new-features-in-java-18.html - especially UTF-8 as default - yeah 🎉

[koppor]

IntelliJ doesn't seem to support Java 18 (yet): https://www.jetbrains.com/help/idea/supported-java-versions.html#2021 - maybe, we should wait until there is an IntelliJ update?

[calixtus]

I agree. There is no hurry.

[koppor]

https://whichjdk.com/ --> UTF-8 as default

Nevertheless, as soon as Eclipse supports JDK18, we should update

[koppor]

Fetcher tests failing is mostly IEEE:

java.io.IOException: Server returned HTTP response code: 503 for URL: https://ieeexplore.ieee.org/document/7421926/