Ensure PromiseLikeOfReactNode is not included in .d.ts files (#63185)

Fixes the compile error on all test runs currently:
https://github.com/vercel/next.js/actions/runs/8243077904/job/22543375810?pr=63167#step:27:345

The root cause is that `.d.ts` files automatically include inferred
return types automatically, in this case `tsc` included React class
component `render()` return types in the `.d.ts` for e.g.
error-boundary.tsx / redirect-boundary.tsx. This is a problem because
yesterday that return type was changed:
https://github.com/DefinitelyTyped/DefinitelyTyped/commit/bf659aefa780a8b61b1636bd0296e0723c96d780#diff-1d64e275d9755825ba[…]520436dbd8e1f1fd9fc66a9
and that change also removes one of the previous types that the `.d.ts`
in Next.js automatically included.

This PR changes the `render()` return type to be explicit instead of
inferred, this makes sure that the `.d.ts` file includes only `:
React.ReactNode` instead of the many types of return values allowed.

The reason I went with the explicit type instead of e.g. upgrading
`@types/react` is that upgrading the types would cause existing
applications that use older versions of `@types/react` would break. The
current change ensures it works in both cases.

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Closes NEXT-2787

packages/next/src/client/components/error-boundary.tsx

@@ -109,7 +109,8 @@ export class ErrorBoundaryHandler extends React.Component<
     this.setState({ error: null })
   }
 
-  render() {
+  // Explicit type is needed to avoid the generated `.d.ts` having a wide return type that could be specific the the `@types/react` version.
+  render(): React.ReactNode {
     if (this.state.error) {
       return (
         <>

packages/next/src/client/components/react-dev-overlay/pages/ErrorBoundary.tsx

@@ -30,7 +30,8 @@ export class ErrorBoundary extends React.PureComponent<
     }
   }
 
-  render() {
+  // Explicit type is needed to avoid the generated `.d.ts` having a wide return type that could be specific the the `@types/react` version.
+  render(): React.ReactNode {
     // The component has to be unmounted or else it would continue to error
     return this.state.error ||
       (this.props.globalOverlay && this.props.isMounted) ? (

packages/next/src/client/components/redirect-boundary.tsx

@@ -58,7 +58,8 @@ export class RedirectErrorBoundary extends React.Component<
     throw error
   }
 
-  render() {
+  // Explicit type is needed to avoid the generated `.d.ts` having a wide return type that could be specific the the `@types/react` version.
+  render(): React.ReactNode {
     const { redirect, redirectType } = this.state
     if (redirect !== null && redirectType !== null) {
       return (