fix: Improve legacyRoot error message

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, and you need help with upgrading your app, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide.`,
     )
   })
 
@@ -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, and you need help with upgrading your app, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide.`,
     )
   })
 })

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, and you need help with upgrading your app, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide.`,
   )
 })

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, and you need help with upgrading your app, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide.',
     )
     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, and you need help with upgrading your app, visit https://react.dev/blog/2022/03/08/react-18-upgrade-guide.',
     )
     Error.captureStackTrace(error, renderHook)
     throw error