Revert "[8.9](backport #4030) Remove BETA mention from UP"

[dedemorton]

Reverts elastic/observability-docs#4031

8.9 is still marked as Beta in the main docs, so I am reverting this backport.

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[dedemorton]

This PR confirms that UP beta was 8.10: https://github.com/elastic/observability-docs/pull/3147