docs(forms): replace 'legacy' with 'existing' in migration guide and navigation

Updates terminology from 'Legacy Forms' to 'Existing Forms' in:
- Migration guide documentation
- Navigation entries label

Backport of #66969

adev/src/app/routing/navigation-entries/index.ts

@@ -500,7 +500,7 @@ export const DOCS_SUB_NAVIGATION_DATA: NavigationItem[] = [
             status: 'new',
           },
           {
-            label: 'Migrating from Legacy Forms',
+            label: 'Migrating from Reactive Forms',
             path: 'guide/forms/signals/migration',
             contentPath: 'guide/forms/signals/migration',
             category: 'Signal Forms',

adev/src/content/guide/forms/signals/migration.md

@@ -1,7 +1,7 @@
 # Migrating existing forms to Signal Forms
 
 This guide provides strategies for migrating existing codebases to Signal Forms, focusing on interoperability with
-legacy Reactive Forms.
+existing Reactive Forms.
 
 ## Top-down migration using `compatForm`
 
@@ -10,7 +10,7 @@ controls that involve:
 
 - Complex asynchronous logic.
 - Intricate RxJS operators that are not yet ported.
-- Integration with legacy third-party libraries.
+- Integration with existing third-party libraries.
 
 ### Integrating a `FormControl` into a signal form
 
@@ -24,7 +24,7 @@ import {signal} from '@angular/core';
 import {FormControl, Validators} from '@angular/forms';
 import {compatForm} from '@angular/forms/signals/compat';
 
-// 1. Existing legacy control with a specialized validator
+// 1. Existing control with a specialized validator
 const passwordControl = new FormControl('', {
   validators: [Validators.required, enterprisePasswordValidator()],
   nonNullable: true,
@@ -33,7 +33,7 @@ const passwordControl = new FormControl('', {
 // 2. Wrap it inside your form state signal
 const user = signal({
   email: '',
-  password: passwordControl, // Nest the legacy control directly
+  password: passwordControl, // Nest the existing control directly
 });
 
 // 3. Create the form
@@ -45,7 +45,7 @@ console.log(f.password().value()); // Current value of passwordControl
 
 // Reactive state is proxied automatically
 const isPasswordValid = f.password().valid();
-const passwordErrors = f.password().errors(); // Returns CompatValidationError if the legacy validator fails
+const passwordErrors = f.password().errors(); // Returns CompatValidationError if the existing validator fails
 ```
 
 In the template, use standard reactive syntax by binding the underlying control:
@@ -83,14 +83,15 @@ In the template, use standard reactive syntax by binding the underlying control:
 
 ### Integrating a `FormGroup` into a signal form
 
-You can also wrap an entire `FormGroup`. This is common when a reusable sub-section of a form—such as an **Address Block**—is still managed by legacy Reactive Forms.
+You can also wrap an entire `FormGroup`. This is common when a reusable sub-section of a form—such as an
+**Address Block**—is still managed by existing Reactive Forms.
 
 ```typescript
 import {signal} from '@angular/core';
 import {FormGroup, FormControl, Validators} from '@angular/forms';
 import {compatForm} from '@angular/forms/signals/compat';
 
-// 1. A legacy address group with its own validation logic
+// 1. An existing address group with its own validation logic
 const addressGroup = new FormGroup({
   street: new FormControl('123 Angular Way', Validators.required),
   city: new FormControl('Mountain View', Validators.required),
@@ -109,7 +110,7 @@ const f = compatForm(checkoutModel, (p) => {
 ```
 
 The `shippingAddress` field acts as a branch in your Signal Form tree. You can bind these nested controls in your
-template by accessing the underlying legacy controls via `.control()`:
+template by accessing the underlying existing controls via `.control()`:
 
 ```angular-html
 <form novalidate>
@@ -187,7 +188,7 @@ const passwordControl = new FormControl('password' /** ... */);
 
 const user = signal({
   email: '',
-  password: passwordControl, // Nest the legacy control directly
+  password: passwordControl, // Nest the existing control directly
 });
 
 const form = compatForm(user);