changed Keyword.of to accept List<String> instead of varargs

[siddhantkore]

Closes #13602

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[calixtus]

Changelog entries only for user facing changes

[Siedlerchr]

Not needed, the changelog is user facing and this is only an internal thing

[calixtus]

Careful: Keyword.of does not create multiple Keywords or a List of Keywords, but one hierarchical one. Changes to List is in this case mostly cosmetic and using more modern java style with List safeties.

[siddhantkore]

Hi @calixtus You right. Thanks for pointing this out and for clarification!

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨