[flow] Eliminate a few React.Element type that will be synced to react-native (#30719)

## Summary

Flow will eventually remove the specific `React.Element` type. For most
of the code, it can be replaced with `React.MixedElement` or
`React.Node`.

When specific react elements are required, it needs to be replaced with
either `React$Element` which will trigger a `internal-type` lint error
that can be disabled project-wide, or use
`ExactReactElement_DEPRECATED`.

Fortunately in this case, this one can be replaced with just
`React.MixedElement`.

## How did you test this change?

`flow`

packages/react-native-renderer/src/ReactNativeRenderer.js

@@ -8,7 +8,7 @@
  */
 
 import type {ReactPortal, ReactNodeList} from 'shared/ReactTypes';
-import type {ElementRef, Element, ElementType} from 'react';
+import type {ElementRef, ElementType, MixedElement} from 'react';
 import type {FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
 import type {RenderRootOptions} from './ReactNativeTypes';
 
@@ -117,7 +117,7 @@ function nativeOnCaughtError(
 }
 
 function render(
-  element: Element<ElementType>,
+  element: MixedElement,
   containerTag: number,
   callback: ?() => void,
   options: ?RenderRootOptions,

packages/react-native-renderer/src/ReactNativeTypes.js

@@ -9,7 +9,12 @@
  * @flow strict
  */
 
-import type {ElementRef, ElementType, Element, AbstractComponent} from 'react';
+import type {
+  ElementRef,
+  ElementType,
+  MixedElement,
+  AbstractComponent,
+} from 'react';
 
 export type MeasureOnSuccessCallback = (
   x: number,
@@ -221,7 +226,7 @@ export type ReactNativeType = {
     eventType: string,
   ): void,
   render(
-    element: Element<ElementType>,
+    element: MixedElement,
     containerTag: number,
     callback: ?() => void,
     options: ?RenderRootOptions,
@@ -256,7 +261,7 @@ export type ReactFabricType = {
     eventType: string,
   ): void,
   render(
-    element: Element<ElementType>,
+    element: MixedElement,
     containerTag: number,
     callback: ?() => void,
     concurrentRoot: ?boolean,

packages/react/index.js

@@ -15,6 +15,7 @@ export type AbstractComponent<
 > = React$AbstractComponent<Config, Instance>;
 export type ElementType = React$ElementType;
 export type Element<+C> = React$Element<C>;
+export type MixedElement = React$Element<ElementType>;
 export type Key = React$Key;
 export type Ref<C> = React$Ref<C>;
 export type Node = React$Node;