I'd prefer that December be 5.14 because the Jenkins project is not ready to abandon Java 8 yet and I believe that JGit 6.0 plans to drop Java 8.
I don't think there's anything forcing Jenkins or Jenkins plug-ins to use JGit 6.0 if they currently use 5.x and that works. So I don't quite understand the reasoning here.
Personally, either is fine with me. When we do JGit 6.0 Java 11 based, EGit 6.0 will follow suit and will bump its baseline to Eclipse 2020-09. (First Eclipse version that required Java 11.)