docs: sampling rate of 0.0 does not work with apm server >= 8.0

When APM agents detect APM Server version `>= 8.0`, they do not send unsampled transactions, as defined in the [spec](https://github.com/elastic/apm/blob/main/specs/agents/tracing-sampling.md#non-sampled-transactions).
The spec also defines that the APM Server does not calculcate metrics if sampling rate is set to `0`. So in this scenario, neither `transaction` nor `transaction metric` documents will exist in Elasticsearch.

In comparison, in previous versions < 8, the APM Server would keep all transaction documents, and throw away all span documents, if sampling was set to `0`.

While the behavior change is ok for a major version upgrade, the documentation is lacking. The apm agent documents need to clearly state that a sampling rate of `0` is only supported when using APM Server of versions up to `7.17`. Right now, there is no indication of this in (most) of the apm agent docs. This can lead to missing data on users deployment when upgrading.