Replace deprecated jwt-easy dependency

web-token/jwt-easy is deprecated, and web-token/jwt-library should be used instead (or a subset of it's exported packages.

The library requires PHP 8.1, so it would likely also involve dropping Drupal 9 support, or releasing this under a new major.

Unsure about what the test controller asserted, but I tried to recreate the logic using the exports available in jwt-library.