Update _app.js to use a function component. (#16683)

This PR updates the `_app.js` file to remove the `next/app` import. It's not needed with the latest versions of Next

Author: pkrawc

examples/with-styled-components/pages/_app.js

@@ -1,4 +1,3 @@
-import App from 'next/app'
 import { ThemeProvider } from 'styled-components'
 
 const theme = {
@@ -7,13 +6,10 @@ const theme = {
   },
 }
 
-export default class MyApp extends App {
-  render() {
-    const { Component, pageProps } = this.props
-    return (
-      <ThemeProvider theme={theme}>
-        <Component {...pageProps} />
-      </ThemeProvider>
-    )
-  }
+export default function App({ Component, pageProps }) {
+  return (
+    <ThemeProvider theme={theme}>
+      <Component {...pageProps} />
+    </ThemeProvider>
+  )
 }