Workflows docs: add serverless: ga label to nested-inputs applies-switch when the change ships in serverless

Sub-issue of [docs-content-internal#1151](https://github.com/elastic/docs-content-internal/issues/1151) (Workflows docs maturity META).

**Blocked on engineering shipping the nested-inputs schema change in serverless.** No docs work to do today; this issue exists to remind us to update the applies-switch labels when that change goes live.

## Context

In 9.5 stack, `inputs:` moves from the workflow root to inside the `manual` trigger. The schema change is **not yet live in serverless**. Per Tinsae's PR #6669 review:

> existing workflows are fine, even though the editor highlights this as an error. They'll continue to work. New workflows, though, in serverless and in 9.5 can't be enabled without moving the input def under the manual trigger

When the change ships in serverless, the workflow docs need an applies-switch label sweep so the new (nested) form is correctly marked as applying to serverless too.

## What to update

PR [#6669](https://github.com/elastic/docs-content/pull/6669) introduced the pattern. Any page that uses an applies-switch for the inputs placement currently labels the two items like this:

```markdown
:::{applies-item} { stack: ga 9.4, serverless: ga }
[Legacy: root-level inputs]
:::

:::{applies-item} stack: ga 9.5+
[New: nested under manual]
:::
```

When serverless catches up, change the new-form label to `{ stack: ga 9.5+, serverless: ga }` and the legacy-form label to `stack: ga 9.4` only (since serverless will no longer accept the legacy form for new workflows).

## Tasks (when unblocked)

- [ ] Confirm with engineering / @tinnytintin10 that nested-inputs is live in serverless.
- [ ] Sweep [`anatomy.md`](https://github.com/elastic/docs-content/blob/main/explore-analyze/workflows/authoring-techniques/anatomy.md) and any pages added by sub-issue #6811: update the applies-item labels as described above.
- [ ] Verify behavior on existing serverless workflows that still use top-level `inputs:` — note in the docs whether they continue to run or need migration.

## Resources

- Companion sweep issue: [#6811](https://github.com/elastic/docs-content/issues/6811) (apply nested-inputs uniformly across all workflows docs).
- Engineering tracking issue for the legacy retirement: [elastic/security-team#16526](https://github.com/elastic/security-team/issues/16526).
- Schema: [`kbn-workflows/spec/schema/triggers/manual_trigger_schema.ts`](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-workflows/spec/schema/triggers/manual_trigger_schema.ts).

## Point of contact

**Main contact:** @benironside

**Stakeholders:** @tinnytintin10