Update text for optional FLAG part

What does this MR do and why?

Replaces

The feature is not ready for production use.

with

This feature is not ready for production use.

in the gitlab repo.

Related guidance

• https://docs.gitlab.com/ee/development/documentation/feature_flags.html#use-a-note-to-describe-the-state-of-the-feature-flag

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

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