Remove BETA mention from UP

[lucabelluccini]

This PR will need to be backported to all versions back to 8.9. Please check with PM for confirmation.

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[mergify[bot]]

This pull request does not have a backport label. Could you fix it @lucabelluccini? 🙏 To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit NOTE: backport-skip has been added to this pull request.

[dedemorton]

I'll go ahead and squash/merge this to save time since it's a small change. Thanks!

[dedemorton]

The following PR confirms that UP went GA in 8.10, so I have reverted the 8.9 backport. https://github.com/elastic/observability-docs/pull/3147