bug: timer threads created in WebsocketGraphQLTransportWSProtocolHandler persist after session closes

Expected behavior

Timer threads cancel after session has been closed and removed from contexts

Actual behavior

Timer threads remain after sessions are closed, increasing the jvm thread count. Timer threads are being created here:

dgs-framework/graphql-dgs-subscriptions-websockets/src/main/kotlin/com/netflix/graphql/dgs/subscriptions/websockets/WebsocketGraphQLTransportWSProtocolHandler.kt

Line 81 in 38f483f

timer.schedule(timerTask, connectionInitTimeout.toMillis())

Steps to reproduce

Open a ws connection and send a "connection_init" + "subscribe" message using graphql-transport-ws. A timer thread should have been created. Close the connection and the threads remain.

opened #1808