fix: Improve legacyRoot error message (#1301)

Author: eps1lon

src/__tests__/render.js

@@ -234,7 +234,7 @@ describe('render API', () => {
     expect(() => {
       render(<div />, {legacyRoot: true})
     }).toThrowErrorMatchingInlineSnapshot(
-      `\`legacyRoot: true\` is not supported in this version of React. Please use React 18 instead.`,
+      `\`legacyRoot: true\` is not supported in this version of React. If your app runs React 19 or later, you should remove this flag. If your app runs React 18 or earlier, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide for upgrade instructions.`,
     )
   })
 
@@ -259,7 +259,7 @@ describe('render API', () => {
     expect(() => {
       render(ui, {container, hydrate: true, legacyRoot: true})
     }).toThrowErrorMatchingInlineSnapshot(
-      `\`legacyRoot: true\` is not supported in this version of React. Please use React 18 instead.`,
+      `\`legacyRoot: true\` is not supported in this version of React. If your app runs React 19 or later, you should remove this flag. If your app runs React 18 or earlier, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide for upgrade instructions.`,
     )
   })
 })

src/__tests__/renderHook.js

@@ -108,6 +108,6 @@ testGateReact19('legacyRoot throws', () => {
       },
     ).result
   }).toThrowErrorMatchingInlineSnapshot(
-    `\`legacyRoot: true\` is not supported in this version of React. Please use React 18 instead.`,
+    `\`legacyRoot: true\` is not supported in this version of React. If your app runs React 19 or later, you should remove this flag. If your app runs React 18 or earlier, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide for upgrade instructions.`,
   )
 })

src/pure.js

@@ -209,7 +209,9 @@ function render(
 ) {
   if (legacyRoot && typeof ReactDOM.render !== 'function') {
     const error = new Error(
-      '`legacyRoot: true` is not supported in this version of React. Please use React 18 instead.',
+      '`legacyRoot: true` is not supported in this version of React. ' +
+        'If your app runs React 19 or later, you should remove this flag. ' +
+        'If your app runs React 18 or earlier, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide for upgrade instructions.',
     )
     Error.captureStackTrace(error, render)
     throw error
@@ -274,7 +276,9 @@ function renderHook(renderCallback, options = {}) {
 
   if (renderOptions.legacyRoot && typeof ReactDOM.render !== 'function') {
     const error = new Error(
-      '`legacyRoot: true` is not supported in this version of React. Please use React 18 instead.',
+      '`legacyRoot: true` is not supported in this version of React. ' +
+        'If your app runs React 19 or later, you should remove this flag. ' +
+        'If your app runs React 18 or earlier, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide for upgrade instructions.',
     )
     Error.captureStackTrace(error, renderHook)
     throw error