[STAC-24731] Rename platform otel collector to otel-receiver

[VioletCranberry]

Summary

The platform's OTLP ingest component was named after the upstream OpenTelemetry Collector, so readers repeatedly confused it with the collector they deploy for their own applications. The Helm chart renames it to otel-receiver (StackVista/helm-charts-internal#156); this updates the docs to match, and states the distinction explicitly.

What changed

Renamed references to the platform component:

• setup/otel/otlp-apis.adoc - in-cluster service endpoint is now suse-observability-otel-receiver
• setup/install-stackstate/advanced-troubleshooting.adoc - pod is suse-observability-otel-receiver-0, and the component is listed as OTLP Receiver rather than OpenTelemetry Collector
• setup/install-stackstate/kubernetes_openshift/ingress.adoc - values key and serviceName in all ingress and gateway examples
• use/security/self-signed-certificates.adoc - values key in both ingress examples

Added guidance:

• A 2.10.4 entry in version-specific-upgrade-instructions.adoc covering the renamed values key and the renamed Kubernetes resources. It calls out that the old key now causes a hard failure rather than being silently ignored, and that app.kubernetes.io/name changes from opentelemetry-collector to otel-receiver, which user-authored NetworkPolicy / ServiceMonitor / PrometheusRule selectors and dashboards may depend on.
• A note on the OTLP protocol page stating that otel-receiver is the platform's own ingest endpoint and not the collector a user deploys, since conflating the two is what prompted the rename.

Deliberately unchanged

• All setup/otel/ pages about deploying your own collector. The upstream chart, image, repository and SUSE Application Collection references legitimately use the opentelemetry-collector name, as does job_name: opentelemetry-collector in user collector configs.
• v2.10.0-helm.adoc. It is a version-pinned migration guide for a release that shipped under the old name; editing it would make it inaccurate.
• migrate-from-6.adoc and migrate-from-6-new.adoc. The opentelemetry-collector: block in each edits the legacy StackState 6.x chart's own values, not ours. Worth a second opinion: step 1 of that procedure tells users to copy their ingress config into the SUSE Observability values, where the key would now need to be otel-receiver:. The guide never shows that target snippet, so nothing is concretely wrong, but a docs owner may want to add a pointer.
• Locales other than en. Following the convention in recent commits, which touch en only.

Open question

The upgrade note is filed under 2.10.4, matching the chart's current 2.10.4-pre train. If the rename is held for 2.11.0 instead, that heading needs to move.

Test plan

• make local builds cleanly; the only asciidoctor warnings are pre-existing invalid references in the de and es locales
• Verified in the rendered HTML that the new upgrade entry, its nested YAML block and the admonition all render, and that the xref to the collector page resolves
• Docs review of the wording, and confirmation of the target release version

Relates to STAC-24731

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