[Serverless] Updates D4C badge from tech preview to beta

[benironside]

Fixes #5454 by updating the tech preview badge to beta for D4C docs (serverless)

Previews: D4C overview, get started with D4C

[github-actions]

A documentation preview will be available soon.

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[mergify]

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

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

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[joepeeples]

Soooo... the DocBadge at the top of every serverless docs page should remain as technical preview, because all of serverless is still considered technical preview.

We never really worked out a great way to handle release states for individual features within serverless, but one option is to add a callout that identifies the feature's state. I think the syntax I suggested should do this, but might want to test it first.