docs/otel-direct.asciidoc has outdated configuration

[mx-psi]

The Collector example in https://github.com/elastic/apm-server/blob/8.10/docs/otel-direct.asciidoc is outdated in two ways:

• It uses the logging exporter instead of the recommended replacement, the debug exporter
• It uses the legacy ${..} environment variable syntax, instead of the new ${env:...} syntax

It should be updated to avoid using legacy features

[axw]

Thanks for reporting this @mx-psi!

[axw]

This has been on pause while some work was happening to centralise docs - will be fixed by elastic/observability-docs#3518