Add 'Enhanced' as an issue doc option

What does this MR do?

At the moment, when we have a "new" feature, the style guide suggests that we say "Introduced in GitLab X.Y"

However, we don't have direction on what happens when we enhance a feature.

• In some cases, it's possible to include [Introduced in] with the new content within a section
• What happens when the content is changing?

I therefore propose that we include [Enhanced] as an option, as detailed in this MR.

Based on !49733 (merged)

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
  • Ensure that the product tier badge is added to doc's h1.
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    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
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Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

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