"Configure frozen searchable snapshots" section is stale — Kibana UI now supports it

## Summary

[`manage-data/lifecycle/data-stream/tutorial-update-existing-data-stream.md`](https://github.com/elastic/docs-content/blob/main/manage-data/lifecycle/data-stream/tutorial-update-existing-data-stream.md), section **"Configure frozen searchable snapshots"** (`#configure-dlm-searchable-snapshots`), currently says:

> You cannot use the **Index Management** tools in {{kib}} to accomplish this step. Use the [PUT data stream lifecycle API]...

This is no longer accurate. The current **Edit data lifecycle** flyout (Index Management → Data Streams → a data stream's details → **Actions** → **Edit data lifecycle** → **Data stream lifecycle** method) now includes a **Frozen phase** card that configures `frozen_after` directly in the UI. It's gated on:
- An Enterprise license
- A default snapshot repository being configured (shows a **Default repository required** message otherwise — confirmed live by @florent-leborgne)

## Where this was found

Found while verifying [PR #7901](https://github.com/elastic/docs-content/pull/7901) (issue #7637) against the live product. Not fixed there since it's a separate, pre-existing section this PR doesn't otherwise touch.

## Source pointers (Kibana, `elastic/kibana`)

- `x-pack/platform/plugins/shared/index_management/public/application/components/data_lifecycle/dlm_phases_selector/dlm_phases_selector.tsx` — renders the Frozen phase card when `!serverless && enterprise && createDefaultRepositoryUrl && (hasDefaultSnapshotRepository || canCreateDefaultSnapshotRepository || alreadyActive)`.
- `.../dlm_phases_selector/components/frozen_phase_card.tsx` — the Frozen phase card itself.

This Frozen-phase-in-UI capability is part of the broader "Edit data lifecycle" redesign (Stack 9.5.0, Kibana PRs [#271930](https://github.com/elastic/kibana/pull/271930) / [#275740](https://github.com/elastic/kibana/pull/275740)), but I haven't verified the Frozen phase card specifically was present from 9.5.0 or landed slightly later in the same cycle — worth confirming before setting an `applies_to` floor.

## Suggested fix

Update the section to document the UI path (with its Enterprise + default-repository prerequisites) as the primary option, keeping the API steps as an alternative or for scripting/automation use cases. Preserve the cumulative-docs distinction if the UI path only applies from a specific Stack version.

Open manage-data/lifecycle/data-stream/tutorial-update-existing-data-stream.md and review the “Configure frozen searchable snapshots” section, then compare its guidance with the referenced Kibana frozen_phase_card.tsx behavior. Confirm the applicable Stack version and document the UI path, Enterprise and default-repository prerequisites, while retaining the API option and cumulative-docs distinction.

Written by the indexing model from the issue text.