Update to JDK21 - both JDK and language level

[koppor]

Fixes https://github.com/JabRef/jabref-issue-melting-pot/issues/309

No JDK 21u1 with Adoptium as of today (adoptium/temurin#6)

I checked available SDKs. I read through https://bell-sw.com/blog/oracle-java-alternatives-comparison-of-openjdk-distributions/#mcetoc_1h197r93f2ih and https://github.com/SAP/SapMachine/wiki/Differences-between-SapMachine-and-OpenJDK. "Liberica" came out best for me. Still using Temurin in the docs.

koppor@DESKTOP-KAK953S:/mnt/c/Users/koppor$ sdk list java | grep 21                       
 Corretto      |     | 21           | amzn    |            | 21-amzn                      
               |     | 21.0.1       | amzn    |            | 21.0.1-amzn                  
               |     | 11.0.21      | amzn    |            | 11.0.21-amzn                 
 GraalVM CE    |     | 21           | graalce |            | 21-graalce                   
 GraalVM Oracle|     | 21           | graal   |            | 21-graal                     
               |     | 21.0.1       | graal   |            | 21.0.1-graal                 
               |     | 21           | open    |            | 21-open                      
               |     | 21.ea.35     | open    |            | 21.ea.35-open                
               |     | 21.0.1       | open    |            | 21.0.1-open                  
 Liberica      |     | 21.fx        | librca  |            | 21.fx-librca                 
               |     | 21.0.1.fx    | librca  |            | 21.0.1.fx-librca             
               |     | 21.0.1       | librca  |            | 21.0.1-librca                
               |     | 21           | librca  |            | 21-librca                    
               |     | 11.0.21.fx   | librca  |            | 11.0.21.fx-librca            
               |     | 11.0.21      | librca  |            | 11.0.21-librca               
               |     | 23.1.r21     | nik     |            | 23.1.r21-nik                 
               |     | 23.1.r21     | mandrel |            | 23.1.r21-mandrel             
 Microsoft     |     | 21           | ms      |            | 21-ms                        
 Oracle        |     | 21.0.1       | oracle  |            | 21.0.1-oracle                
               |     | 21           | oracle  |            | 21-oracle                    
 SapMachine    |     | 21           | sapmchn |            | 21-sapmchn                   
               |     | 21.0.1       | sapmchn |            | 21.0.1-sapmchn               
               |     | 11.0.21      | sapmchn |            | 11.0.21-sapmchn              
 Temurin       |     | 21           | tem     |            | 21-tem                       
               |     | 11.0.21      | tem     |            | 11.0.21-tem                  
 Zulu          |     | 21           | zulu    |            | 21-zulu                      
               |     | 21.crac      | zulu    |            | 21.crac-zulu                 
               |     | 21.fx        | zulu    |            | 21.fx-zulu                   
               |     | 21.0.1       | zulu    |            | 21.0.1-zulu                  
               |     | 11.0.21      | zulu    |            | 11.0.21-zulu               

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]

About dialog looks good:

[tobiasdiez]

Good job everyone! Nice to see that the work on fixing the stupid JDK bug paid off and came to a nice conclusion.

[github-actions]

Your code currently does not meet JabRef's code guidelines. The tool reviewdog already placed comments on GitHub to indicate the places. See the tab "Files" in you PR. Please carefully follow the setup guide for the codestyle. Afterwards, please run checkstyle locally and fix the issues.

More information on code quality in JabRef is available at https://devdocs.jabref.org/getting-into-the-code/development-strategy.html.

[github-actions]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.