[DOCS-2906] Indicate deprecation of opentelemetry-exporters-datadog

[knarewski]

What does this PR do?

Updates the documentation to indicate that opentelemetry-exporters-datadog gem is deprecated

Motivation

The documentation states that it's in beta, which conflicts with the gem's Readme.

Preview

• content/en/tracing/setup_overview/open_standards/ruby.md#opentelemetry

Notes

• deprecation PR

---

Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[kayayarai]

Verifying with engineering/support - DOCS-2906

[kayayarai]

Datadog's support for this Ruby exporter is completely replaced by the Datadog Exporter for OTel Collector, so this section of the Ruby page has been removed.