Add deprecation warning for findDOMNode

This is removed in version 19. We already warned inside of Strict Mode
but this adds the warning everywhere.

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

@@ -238,10 +238,22 @@ function legacyRenderSubtreeIntoContainer(
   return getPublicRootInstance(root);
 }
 
+let didWarnAboutFindDOMNode = false;
+
 export function findDOMNode(
   componentOrElement: Element | ?React$Component<any, any>,
 ): null | Element | Text {
   if (__DEV__) {
+    if (!didWarnAboutFindDOMNode) {
+      didWarnAboutFindDOMNode = true;
+      console.error(
+        'findDOMNode is deprecated and will be removed in the next major ' +
+          'release. Instead, add a ref directly to the element you want ' +
+          'to reference. Learn more about using refs safely here: ' +
+          'https://reactjs.org/link/strict-mode-find-node',
+      );
+    }
+
     const owner = (ReactCurrentOwner.current: any);
     if (owner !== null && owner.stateNode !== null) {
       const warnedAboutRefsInRender = owner.stateNode._warnedAboutRefsInRender;

scripts/jest/shouldIgnoreConsoleError.js

@@ -31,7 +31,8 @@ module.exports = function shouldIgnoreConsoleError(format, args) {
           'uses the legacy childContextTypes API which is no longer supported and will be removed'
         ) !== -1 ||
         format.indexOf('ReactDOMTestUtils is deprecated') !== -1 ||
-        format.indexOf('`ReactDOMTestUtils.act` is deprecated') !== -1
+        format.indexOf('`ReactDOMTestUtils.act` is deprecated') !== -1 ||
+        format.indexOf('findDOMNode is deprecated and will be removed') !== -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