feat!: Upgrade to React 19 (#21523)

Removes the React 19 feature flag and always uses React 19.

Author: Artur-

flow-server/src/main/java/com/vaadin/experimental/FeatureFlags.java

@@ -78,10 +78,6 @@ public class FeatureFlags implements Serializable {
             "https://github.com/vaadin/platform/issues/7173", true,
             "com.vaadin.flow.component.masterdetaillayout.MasterDetailLayout");
 
-    public static final Feature REACT19 = new Feature(
-            "React 19 (default in Vaadin 25)", "react19",
-            "https://react.dev/blog/2024/12/05/react-19", true, null);
-
     public static final Feature ACCESSIBLE_DISABLED_BUTTONS = new Feature(
             "Accessible disabled buttons", "accessibleDisabledButtons",
             "https://github.com/vaadin/web-components/issues/4585", true, null);
@@ -127,7 +123,6 @@ public FeatureFlags(Lookup lookup) {
         features.add(new Feature(FLOW_FULLSTACK_SIGNALS));
         features.add(new Feature(COPILOT_EXPERIMENTAL));
         features.add(new Feature(MASTER_DETAIL_LAYOUT_COMPONENT));
-        features.add(new Feature(REACT19));
         features.add(new Feature(ACCESSIBLE_DISABLED_BUTTONS));
         features.add(new Feature(LAYOUT_COMPONENT_IMPROVEMENTS));
         features.add(new Feature(DEFAULT_AUTO_RESPONSIVE_FORM_LAYOUT));

flow-server/src/main/java/com/vaadin/flow/server/frontend/NodeUpdater.java

@@ -307,10 +307,6 @@ Map<String, String> getDefaultDependencies() {
         if (options.isReactEnabled()) {
             dependencies
                     .putAll(readDependencies("react-router", "dependencies"));
-            if (options.getFeatureFlags().isEnabled(FeatureFlags.REACT19)) {
-                dependencies
-                        .putAll(readDependencies("react19", "dependencies"));
-            }
         } else {
             dependencies
                     .putAll(readDependencies("vaadin-router", "dependencies"));
@@ -377,9 +373,6 @@ Map<String, String> getDefaultDevDependencies() {
         if (options.isReactEnabled()) {
             defaults.putAll(
                     readDependencies("react-router", "devDependencies"));
-            if (options.getFeatureFlags().isEnabled(FeatureFlags.REACT19)) {
-                defaults.putAll(readDependencies("react19", "devDependencies"));
-            }
         }
 
         return defaults;

flow-server/src/main/resources/com/vaadin/flow/server/frontend/dependencies/react-router/package.json

@@ -2,12 +2,12 @@
   "private": true,
   "description": "A list of Flow dependencies when using React router",
   "dependencies": {
-    "react": "18.3.1",
-    "react-dom": "18.3.1",
+    "react": "19.1.1",
+    "react-dom": "19.1.1",
     "react-router": "7.7.1"
   },
   "devDependencies": {
-    "@types/react": "18.3.23",
-    "@types/react-dom": "18.3.7"
+    "@types/react": "19.1.9",
+    "@types/react-dom": "19.1.7"
   }
 }

flow-server/src/main/resources/com/vaadin/flow/server/frontend/dependencies/react19/package.json

@@ -145,31 +145,6 @@ public void getDefaultDependencies_includesAllDependencies() {
         Assert.assertEquals(expectedDependencies, actualDependendencies);
     }
 
-    @Test
-    public void react19UsedWhenFeatureFlagIsOn() {
-        Map<String, String> react18Deps = nodeUpdater.getDefaultDependencies();
-        Map<String, String> react18DevDeps = nodeUpdater
-                .getDefaultDevDependencies();
-        Mockito.when(options.getFeatureFlags().isEnabled(FeatureFlags.REACT19))
-                .thenReturn(true);
-
-        Map<String, String> react19Deps = nodeUpdater.getDefaultDependencies();
-        Map<String, String> react19DevDeps = nodeUpdater
-                .getDefaultDevDependencies();
-
-        Assert.assertTrue(react18Deps.get("react").startsWith("18."));
-        Assert.assertTrue(react18Deps.get("react-dom").startsWith("18."));
-        Assert.assertTrue(react18DevDeps.get("@types/react").startsWith("18."));
-        Assert.assertTrue(
-                react18DevDeps.get("@types/react-dom").startsWith("18."));
-
-        Assert.assertTrue(react19Deps.get("react").startsWith("19."));
-        Assert.assertTrue(react19Deps.get("react-dom").startsWith("19."));
-        Assert.assertTrue(react19DevDeps.get("@types/react").startsWith("19."));
-        Assert.assertTrue(
-                react19DevDeps.get("@types/react-dom").startsWith("19."));
-    }
-
     @Test
     public void getDefaultDevDependencies_includesAllDependencies_whenUsingVite() {
         Map<String, String> defaultDeps = nodeUpdater