limitations.md calls the elasticsearch exporter unsupported, but it's the recommended gateway exporter

`limitations.md` says the `elasticsearch` exporter must not be used with Elastic APM / Observability. That's wrong: it's the recommended exporter for an Elastic Agent gateway on self-managed, ECE, and ECK. The distinction should be **edge vs gateway**, not supported vs unsupported.

## Incorrect

`solutions/observability/apm/opentelemetry/limitations.md:57`

> The [OpenTelemetry Collector exporter for Elastic](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/elasticsearchexporter) (which is different from the legacy exporter mentioned above) is not intended to be used with Elastic APM and Elastic Observability.

`solutions/observability/apm/opentelemetry/upstream-opentelemetry-collectors-language-sdks.md:148` repeats the claim and links to `limitations.md` as its justification:

> Using the [`elasticsearch` exporter](...) instead bypasses Elastic's data processing pipeline and is [not supported for use with {{product.observability}}](...).

Surfaced while working on #6569.

Start with the cited passages at lines 57 and 148 of limitations.md and upstream-opentelemetry-collectors-language-sdks.md, then review the linked exporter context. Update both statements to distinguish edge and gateway deployments, including the documented self-managed, ECE, and ECK gateway use case. Done means the two pages no longer contradict the recommended gateway exporter guidance.

Written by the indexing model from the issue text.