_bleio: remove PacketBuffer.packet_size for 7.0.0

[dhalbert]

In 5.x, after #2799 is merged, PacketBuffer.packet_size is a synonym for PacketBuffer.incoming_packet_length, and is deprecated.

In 6.0.0, it can be dropped, after updating the libraries that use it.

[dhalbert]

.incoming_packet_length was added after 5.3.0, so unless we have another 5.x stable release, and then change the libraries, we can't drop .packet_size and have the libraries be backwards compatible without conditional code. We could defer this to 7.0.0, or put in conditional code.

The Adafruit libraries that use .packet_size now are:

• https://github.com/adafruit/Adafruit_CircuitPython_BLE_Adafruit
• https://github.com/adafruit/Adafruit_CircuitPython_BLE_iBBQ
• https://github.com/adafruit/Adafruit_CircuitPython_BLE_Heart_Rate
• https://github.com/adafruit/Adafruit_CircuitPython_BLE_MIDI

[tannewt]

I'm fine waiting for 7.0.0.