Fix bot instance passing in Dispatcher startup (#1247)

Modified the Dispatcher to remove the "bot" key from workflow_data if found, prior to emitting startup in order to allow manual addition of "bot" to workflow data. This change was necessary as a bot instance is required to start polling, and therefore must be passed correctly. This logic is now tested in the 'test_start_polling' function. The change also includes an update to the changelog.

aiogram/dispatcher/dispatcher.py

@@ -516,6 +516,9 @@ class Dispatcher(Router):
                 **self.workflow_data,
                 **kwargs,
             }
+            if "bot" in workflow_data:
+                workflow_data.pop("bot")
+
             await self.emit_startup(bot=bots[-1], **workflow_data)
             loggers.dispatcher.info("Start polling")
             try: