on_telegram_received(): ignore telegrams that are not GroupValueResponse or GroupValueWrite

[fkusei]

We're using a KNX-BACnet gateway, and something (have not yet found out what, i suspect openHAB) is issuing GroupValueRead telegrams, which leads to this error:

Thu 2026-02-05 08:26:58 UTC knx2mqtt[648]: INFO:root:Address: 11/5/101, DPT Type: DPTTemperature, Payload: <GroupValueRead />
Thu 2026-02-05 08:26:58 UTC knx2mqtt[648]: Traceback (most recent call last):
Thu 2026-02-05 08:26:58 UTC knx2mqtt[648]:   File "/opt/knx2mqtt/src/knx2mqtt", line 77, in extract_payload_from_telegram
Thu 2026-02-05 08:26:58 UTC knx2mqtt[648]:     value = dpt_class.from_knx(payload.value)
Thu 2026-02-05 08:26:58 UTC knx2mqtt[648]: AttributeError: 'GroupValueRead' object has no attribute 'value'
Thu 2026-02-05 08:26:58 UTC knx2mqtt[648]: INFO:root:Topic: knx/headquarter/11/5/101, Payload: None

This PR fixes that issue by ignoring all telegrams that are not GroupValueResponse or GroupValueRead.