[Feature Request] Restore support for MQTT 3.1/3.1.1 in aiomqtt 3.x or clearly document dropped versions

[a76yyyy]

Hi, thank you for your work on aiomqtt!

I noticed that aiomqtt 3.x has dropped support for MQTT 3.1 and 3.1.1. In practice, there are still legacy devices and brokers that only support these protocol versions.

• Would it be possible to restore support for MQTT 3.1 and 3.1.1 in aiomqtt 3.x?
• If restoring support is not planned, could you please add a prominent note in the README stating that 3.x no longer supports MQTT 3.1 and 3.1.1 and only supports MQTT 5.0? This will help other users avoid upgrade issues.

Thank you!