Upgrade JJWT to more recent version

[ryanrdoherty]

JJWT is now at 0.12.5 (up from 0.11.5, which we upgraded to during bearer token enhancements). It does offer some new features which could cut down on our custom code, but has a moderately different API and so upgrading is a bit of a project to figure out the new methods/procedures for doing what we need to do, esp regarding signatures and signature algorithms. Should probably do this at some point (it may be beyond 0.12.5 by then), but not an emergency.

https://github.com/jwtk/jjwt
https://mvnrepository.com/artifact/io.jsonwebtoken/jjwt-api