docs migration guide: WiFiServer changes (#9458)

Co-authored-by: Lucas Saavedra Vaz <[email protected]>

docs/en/migration_guides/2.x_to_3.0.rst

@@ -205,3 +205,5 @@ Functional changes
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 * In Arduino (and other frameworks) the method named ``flush()`` is intended to send out the transmit buffer content. ``WiFiClient`` and ``WiFiUDP`` method ``flush()`` won't clear the receive buffer anymore. A new method called ``clear()`` is now used for that. Currently ``flush()`` does nothing in ``WiFiClient``, ``WiFiClientSecure`` and ``WiFiUDP``.
+* ``WiFiServer`` has functions ``accept()`` and ``available()`` with the same functionality. In Arduino, ``available()`` should work differently so it is now deprecated.
+* ``WiFiServer`` had unimplemented write functions inherited from ``Print`` class. These are now removed. Also unimplemented method ``stopAll()`` is removed. The methods were unimplemented because WiFiServer doesn't manage connected WiFiClient objects for print-to-all-clients functionality.