socket-mode 1.x: explicit server disconnect during 'connecting' state not handled

[filmaj]

          @filmaj I seem to be hitting the following now on 1.3.5:

[INFO]  socket-mode:SocketModeClient:0 Going to establish a new connection to Slack ...
/home/patrick/slack/node_modules/finity/lib/core/StateMachine.js:76
      throw new Error('Unhandled event \'' + event + '\' in state \'' + this.currentState + '\'.');
            ^

Error: Unhandled event 'server explicit disconnect' in state 'connecting'.
    at StateMachine.handleUnhandledEvent (/home/patrick/slack/node_modules/finity/lib/core/StateMachine.js:76:13)
    at /home/patrick/slack/node_modules/finity/lib/core/HierarchicalStateMachine.js:79:33
    at TaskScheduler.execute (/home/patrick/slack/node_modules/finity/lib/core/TaskScheduler.js:29:7)
    at TaskScheduler.enqueue (/home/patrick/slack/node_modules/finity/lib/core/TaskScheduler.js:19:12)
    at HierarchicalStateMachine.handle (/home/patrick/slack/node_modules/finity/lib/core/HierarchicalStateMachine.js:72:24)
    at SocketModeClient.onWebSocketMessage (/home/patrick/slack/node_modules/@slack/socket-mode/dist/SocketModeClient.js:608:31)
    at WebSocket.onMessage (/home/patrick/slack/node_modules/ws/lib/event-target.js:132:16)
    at WebSocket.emit (node:events:518:28)
    at Receiver.receiverOnMessage (/home/patrick/slack/node_modules/ws/lib/websocket.js:1068:20)
    at Receiver.emit (node:events:518:28)

With debug logging it shows:

[DEBUG]  socket-mode:SocketModeClient:0 Received a message on the WebSocket: {"type":"disconnect","reason":"too_many_websockets","debug_info":{"host":"applink-2"}}
[DEBUG]  socket-mode:SocketModeClient:0 Received "disconnect" (reason: too_many_websockets) message - will attempt reconnect

Originally posted by @MadrMan in #1654 (comment)

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Any PR to address this should target the socket-mode-1.x branch as this would be a 'maintenance' fix for the now-outdated 1.x line of socket-mode.