docs: document deprecated allowSignalWrites option in v19 update guide

The `allowSignalWrites` option of `effect()` became a no-op and was
deprecated in v19, but the update guide never mentioned it. Applications
upgrading from v18 or earlier still carry the option at their `effect()`
call sites, where it now only produces a dev-mode warning.

Add an entry to the v19 recommendations instructing those applications to
remove it, mirroring the wording of the deprecation notice on
`CreateEffectOptions`.

adev/src/app/features/update/recommendations.ts

@@ -2542,6 +2542,14 @@ export const RECOMMENDATIONS: Step[] = [
     possibleIn: 1900,
     step: '19.0.0-update-fakeasync-to-flush-pending-timers',
   },
+  {
+    action:
+      'Remove the `allowSignalWrites` option from `effect()` calls. The option is deprecated and no longer required, as signal writes inside effects are allowed by default.',
+    level: ApplicationComplexity.Medium,
+    necessaryAsOf: 1900,
+    possibleIn: 1900,
+    step: '19.0.0-remove-allow-signal-writes',
+  },
   {
     action:
       "In the application's project directory, run `ng update @angular/core@20 @angular/cli@20` to update your application to Angular v20.",