Upgrade Jetty

[kurtostfeld]

This slack sdk uses Jetty 9.x which is wildly out of date and EOL. Jetty 12.1 is the current version; that would be best.

[hello-ashleyintech]

Hi, @kurtostfeld! Thank you so much for raising this to our attention. 😄 I will let our internal dev expert for the Java SDK know, but I have also tagged this as a good first issue if you or any other community member would like to give upgrading this a shot!