docs(stacks): stacks is no longer in preview

[defang-sam]

What & why

Prakash confirmed on 2026-09-09 that stacks should no longer be considered a preview feature. This removes the last place that says otherwise.

 :::info
-Stacks is a new feature introduced in Defang CLI v2.4.0 and is currently in preview. We welcome your feedback! Please open an issue on [GitHub](https://github.com/DefangLabs/defang/issues) if you encounter any problems or have suggestions.
+Stacks requires Defang CLI v2.4.0 or later.
 :::

I kept the one fact the banner carried that is still useful — the minimum CLI version. For context, v2.4.0 shipped 2025-11-27 and current is v3.15.1, so the "new feature" framing had outlived itself by about nine months.

Scope — I checked, this really is the only one

Searched defang, defang-docs, defang-mvp and portal for anything describing stacks as preview, beta, experimental, new or not-yet-ready:

Where	Result
defang-docs/docs/concepts/stacks.md:13-15	The one hit. Fixed here
defang CLI — src/cmd/cli/command/stack.go	No preview/experimental wording in any Short/Long help text
Other defang-docs pages mentioning stacks	No preview caveats
Remaining "in preview" strings in defang-docs	All about cloud providers (GCP, DigitalOcean, Azure) or dated blog posts — out of scope, deliberately untouched
defang-mvp website	Only hit is a blog post using "preview a feature before merging" in the ordinary sense

Why it matters beyond this page

Defang Station's new marketing copy — DefangLabs/station#49 — sells "dev, staging and production, each on the cloud and region you choose, in your own account." That is precisely what stacks does, and the page it sends readers to should not call the capability a preview.

Proof

npm run build (with the Go prebuild step) succeeds, and the rendered page is correct:

$ grep -o "Stacks requires Defang CLI v2.4.0 or later." build/docs/concepts/stacks.html
Stacks requires Defang CLI v2.4.0 or later.
$ grep -c "in preview" build/docs/concepts/stacks.html          → 0
$ grep -c "welcome your feedback" build/docs/concepts/stacks.html → 0
$ grep -rl "Stacks is a new feature" build/                      → (no matches)

Two notes on pre-existing conditions, verified against main rather than assumed:

• npm run typecheck reports Cannot find module '@theme/CodeBlock' in src/components/OneClick/index.tsx. Identical on main (stashed and re-ran) — it needs generated Docusaurus types, not this change.
• The build reports two broken anchors, on /docs/concepts/mcp and /docs/providers/aws. Neither is on a page this PR touches.

Risk & rollback

One line of documentation. Revert the single commit.