Add deprecation warning for unmountComponentAtNode

This should have been deprecated in 18.0 alongside the other legacy
DOM APIs like render().

packages/react-dom/src/client/ReactDOMLegacy.js

@@ -393,7 +393,19 @@ export function unstable_renderSubtreeIntoContainer(
   );
 }
 
+let didWarnAboutUnmountComponentAtNode = false;
 export function unmountComponentAtNode(container: Container) {
+  if (__DEV__) {
+    if (!didWarnAboutUnmountComponentAtNode) {
+      didWarnAboutUnmountComponentAtNode = true;
+      console.error(
+        'unmountComponentAtNode is deprecated and will be removed in the ' +
+          'next major release. Switch to the createRoot API. Learn ' +
+          'more: https://reactjs.org/link/switch-to-createroot',
+      );
+    }
+  }
+
   if (!isValidContainerLegacy(container)) {
     throw new Error(
       'unmountComponentAtNode(...): Target container is not a DOM element.',

scripts/jest/shouldIgnoreConsoleError.js

@@ -32,7 +32,9 @@ module.exports = function shouldIgnoreConsoleError(format, args) {
         ) !== -1 ||
         format.indexOf('ReactDOMTestUtils is deprecated') !== -1 ||
         format.indexOf('`ReactDOMTestUtils.act` is deprecated') !== -1 ||
-        format.indexOf('findDOMNode is deprecated and will be removed') !== -1
+        format.indexOf('findDOMNode is deprecated and will be removed') !==
+          -1 ||
+        format.indexOf('unmountComponentAtNode is deprecated') !== -1
       ) {
         // This is a backported warning. In `main`, there's a different warning
         // (and it's fully tested). Not going to bother upgrading all the tests