Switch to Eclipse OpenJ9 JDK v15

[koppor]

Eclipse OpenJ9 is a JVM developed by IBM since the 90s. It has superior performance compared to Oracle's JVM: https://www.eclipse.org/openj9/performance/

We should give it a try in JabRef, too. This PR switchtes to OpenJ9 by using SDKMAN!.

• Change in CHANGELOG.md described (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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[koppor]

Blocked by eclipse-openj9/openj9#11549

[koppor]

This should have been gone to @koppor's repository. Sorry for the noise.