deps: Upgrade to RN v0.65.2!

Note: If you get an error like

  CocoaPods could not find compatible versions for pod "RCT-Folly"

on `yarn` or `pod install` when crossing this commit (checking out a
branch that has it from one that doesn't, or vice versa), run
`pod update RCT-Folly` from `ios/` and try the command again.
Background:
  facebook/react-native#32659 (comment)

In this commit:

- Bump `react` and `react-test-renderer` to follow the RN template
  app

- Adapt to changes in Flow:
  - Two options removed:
      esproposal.optional_chaining,
      esproposal.nullish_coalescing
  - RN no longer clobbers Flow's built-in definitions for `fetch`!
  - Etc.; see comments

- Add react-native-codegen, at ^0.0.7. The RN maintainers moved this
  dep back and forth between the `react-native` NPM package and the
  RN template app. For RN v0.65 it ended up in the template app,
  suggesting that projects using RN v0.65 should depend on it
  directly. So, do that. See discussion:
    #5324 (comment)

.flowconfig

@@ -108,9 +108,6 @@ implicit-inexact-object=error
 [options]
 emoji=true
 
-esproposal.optional_chaining=enable
-esproposal.nullish_coalescing=enable
-
 # The main warning is "Unused suppression comment".
 include_warnings=true
 
@@ -140,4 +137,4 @@ module.file_ext=.ios.js
 exact_by_default=true
 
 [version]
-^0.141.0
+^0.149.0