fix: move internal frontend sources out of deprecated location (#24851) (CP: 25.2) (#24853)

This PR cherry-picks changes from the original PR #24851 to branch 25.2.
---
#### Original PR description
> ## What changed
> 
> `flow-react` and `flow-dnd` shipped their `@JsModule` bundle sources
(`ReactRouterOutletElement.tsx` and `dndConnector.js`) under the
deprecated `META-INF/resources/frontend/` location. With recent builds
this triggers a per-jar deprecation warning.
> 
> - Moved `ReactRouterOutletElement.tsx` and `dndConnector.js` to
`META-INF/frontend/`, the current location for `@JsModule`/`@CssImport`
bundle sources.
> - Updated the `ReactRouterOutlet` entry-point detection in
`FrontendDependencies` to resolve the resource from the new location,
falling back to the legacy `META-INF/resources/frontend/` location for
backwards compatibility with older add-on jars.
> 
> ## Why
> 
> Eliminates the per-jar deprecation warning emitted during the build
while keeping older add-on jars (which still ship sources in the legacy
location) working.
> 
> Fixes #24848

Co-authored-by: totally-not-ai[bot] <290682512+totally-not-ai[bot]@users.noreply.github.com>

Author: vaadin-bot

flow-build-tools/src/main/java/com/vaadin/flow/server/frontend/scanner/FrontendDependencies.java

@@ -66,6 +66,7 @@
 import com.vaadin.flow.theme.NoTheme;
 import com.vaadin.flow.theme.ThemeDefinition;
 
+import static com.vaadin.flow.server.Constants.RESOURCES_FRONTEND_DEFAULT;
 import static com.vaadin.flow.server.frontend.scanner.FrontendClassVisitor.ASSETS;
 import static com.vaadin.flow.server.frontend.scanner.FrontendClassVisitor.DEV;
 import static com.vaadin.flow.server.frontend.scanner.FrontendClassVisitor.VALUE;
@@ -160,8 +161,9 @@ private void computeReactClasses(ClassFinder finder) throws IOException {
         // Add ReactRouterOutlet and adapter as internal so it gets added to the
         // bundle if available.
         try {
-            if (finder.getResource(
-                    "META-INF/resources/frontend/ReactRouterOutletElement.tsx") != null
+            if (finder
+                    .getResource(RESOURCES_FRONTEND_DEFAULT
+                            + "/ReactRouterOutletElement.tsx") != null
                     && !visitedClasses.containsKey(
                             "com.vaadin.flow.component.react.ReactRouterOutlet")) {
                 Class<Object> entryPointClass = finder.loadClass(