docs: remove outdated Actions v2 feature flag references (#12641)

# Which Problems Are Solved

The docs ([Actions v2 concepts
page](https://zitadel.com/docs/concepts/features/actions_v2) and the
Actions testing guides) state that Actions v2 must be enabled via the
"Actions" feature flag. That flag no longer exists — it is `reserved` in
`proto/zitadel/feature/v2/instance.proto` and `system.proto` — and
Actions v2 is available by default. The callout even linked to a
non-existent API field.

# How the Problems Are Solved

- Removed the stale feature-flag callout (and the roadmap teaser
callout) from `concepts/features/actions_v2.mdx`.
- Removed the "Your ZITADEL instance needs to have the actions feature
enabled." prerequisite bullet from the seven
`guides/integrate/actions/testing-*.mdx` guides.

Pure deletions, 17 lines across 8 files.

# Additional Context

The `SetInstanceFeatures` mentions in `hosted-login.mdx` and
`webkeys.mdx` refer to flags that still exist and are untouched.

apps/docs/content/guides/integrate/actions/testing-function.mdx

@@ -12,7 +12,6 @@ This is useful for integrating with other systems which need specific claims in
 Before you start, make sure you have everything set up correctly.
 
 - You need to be at least a ZITADEL [_IAM_OWNER_](/guides/manage/console/administrators)
-- Your ZITADEL instance needs to have the actions feature enabled.
 
 <Callout>
 Note that this guide assumes that ZITADEL is running on the same machine as the target and can be reached via `localhost`.