The "KILLED" value is missing from the QueryExecutionState enum

[OlivierSohn]

Hello,

The enum QueryExecutionState should be updated to contain the KILLED value like in the corresponding C++ enum.

This missing enum value causes this line to throw this exception:

com.arangodb.ArangoDBException: com.arangodb.velocypack.exception.VPackParserException: java.lang.IllegalArgumentException: No enum constant com.arangodb.entity.QueryExecutionState.KILLED

Thanks.

[OlivierSohn]

I'm proposing a PR for this issue. Since I'm a first time contributor, I would need a maintainer to allow the CI workflow to run.