Update Jetty to 9.4.28.v20200408

Description

jetty-9.4.28.v20200408, released 08 April 2020, brings websocket improvements. Notably:
> Setting async timeout on WebSocketClient does not seem to timeout writes

Early tests indicate that this might fix

Environment

None
Fixed
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Assignee

Guus der Kinderen

Reporter

Guus der Kinderen