[Website]: ES OTLP Endpoint for Metrics - Clarify usage

[lucabelluccini]

Type of issue

None

What documentation page is affected

https://www.elastic.co/docs/manage-data/data-store/data-streams/tsds-ingest-otlp

What happened?

This led to confusing decision to some users.

1. make it clear this is tech preview everywhere. We show Stack (Tech Preview) and Self-managed (GA) - which really do not make sense.

2. Improve the messaging this endpoint is not yet meant to be used directly and we strongly recommend to use MOTEL or EDOT Collector GW

Additional info

FYI @strawgate