[Alerting V2] New Slack actions in the Alerting v2 rule form now use the Slack (v2) connector

## Summary

New Slack actions added inline in the Alerting v2 experimental rule form now use the Slack (v2) connector (generating a slack2.sendMessage workflow step) instead of the legacy .slack connector; the action still appears simply as "Slack" in the picker. Per the PR's manual test plan, opening an existing rule that has a legacy .slack inline action will no longer display that action in the rule editor at all (though its underlying workflow is unchanged and keeps dispatching), which could surprise anyone editing an older rule.

**Why this needs docs:** The PR title reads as if the Slack v1 connector itself is being removed, but the diff only removes the '.slack' inline-action option from one specific code path: the Alerting v2 rule form's action picker. The general Slack (v1) connector remains fully valid for classic alerting, cases, and workflows elsewhere, so premiseAccuracy is downgraded to partially-accurate and docsGap is narrowed to the Alerting v2 inline-action surface only.

## Resources

- PR [#278451](https://github.com/elastic/kibana/pull/278451) — [Alerting V2] Use slack2 connector in inline actions, remove slack v1 connector

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Jul 20–Jul 24 |
| **Feature status** | preview |
| **Feature flag** | None — active by default |

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*Created with [Docs Quest Scanner](https://github.com/florent-leborgne/docs-quest-scanner) by @nastasha-solomon*

## Suggested edits

[Notifications and actions for the experimental alerting system](https://www.elastic.co/docs/explore-analyze/alerting/experimental-alerting-system/notifications-actions) > Send notifications or trigger an action
- **What the docs say:** Rules in the experimental alerting system don't send notifications directly. Instead, they produce alert episodes, and you use workflows and action policies to decide what happens next.
- **What to add:** This page only describes building a workflow separately and routing to it with an action policy; it doesn't mention that the rule form also offers a quicker inline action shortcut (Email, Slack, Workflow) that auto-generates the workflow. Add a short note that new inline Slack actions now use the Slack (v2) connector, and that opening a rule with a legacy inline Slack action will no longer show that action in the editor (it keeps dispatching, but isn't visible or editable inline) — keep the existing Slack v1 connector content as-is elsewhere since it's still valid outside this rule-form shortcut. Applies from 9.6.0 (technical preview) and in serverless.