Clarify supported InfluxDB Versions

[Erik1000]

I find it hard to understand which InfluxDB server versions are actually supported by this crate.
I had some luck using InfluxDB2 but the latest InfluxDB server version is 3, which does not work.
The crate should probably declare somewhere at the top of the README which InfluxDB versions are supported...

[msrd0]

IIRC influxdb 1 and 2 (in compatibility mode) should work and are tested in CI. I've read that influxdb 3 wanted to go back to the influxdb 1 protocol, so it could work, but I have not tested compatibility.