Possible API key documentation improvements

[xrmx]

Got some feedback about api key documentation reference here

apm-agent-python/docs/reference/configuration.md

Line 265 in 693318c

### `api_key` [config-api-key]

• I think we may want to remove the tech preview warning since for some platforms it's the only way of authentication
• We may want to suggest to get the api key from kibana instead of the apm-server command

[simitt]

We may want to suggest to get the api key from kibana instead of the apm-server command

The APM Server CLI tool for creating API Keys was never GA and has been deprecated since 8.6, and finally removed in 9.0. Please take a look at Create an API key in the Applications UI link from the apm-server docs - likely best to link to the same.