Bump minimum whatwg-fetch 3.0.0 -> 3.6.20

Summary:
Bump the `package.json` minimum of `whatwg-fetch` to the latest `3.6.20`.

This version is already 3 years old and was already within the `^3.0.0` range, so essentially every OSS user is already using it - as backed up by its download figures: https://www.npmjs.com/package/whatwg-fetch?activeTab=versions

Bumping for clarity ahead of vendoring 3.6.20

Changelog:
[Internal]

Reviewed By: huntie

Differential Revision: D99850877

Author: robhogan

packages/react-native/package.json

@@ -191,7 +191,7 @@
     "semver": "^7.1.3",
     "stacktrace-parser": "^0.1.10",
     "tinyglobby": "^0.2.15",
-    "whatwg-fetch": "^3.0.0",
+    "whatwg-fetch": "^3.6.20",
     "ws": "^7.5.10",
     "yargs": "^17.6.2"
   },

yarn.lock

@@ -9368,7 +9368,7 @@ webidl-conversions@^3.0.0:
   resolved "https://registry.yarnpkg.com/webidl-conversions/-/webidl-conversions-3.0.1.tgz#24534275e2a7bc6be7bc86611cc16ae0a5654871"
   integrity sha512-2JAn3z8AR6rjK8Sm8orRC0h/bcl/DqL7tRPdGZ4I1CjdF+EaMLmYxBHyXuKL849eucPFhvBoxMsflfOb8kxaeQ==
 
-whatwg-fetch@^3.0.0:
+whatwg-fetch@^3.6.20:
   version "3.6.20"
   resolved "https://registry.yarnpkg.com/whatwg-fetch/-/whatwg-fetch-3.6.20.tgz#580ce6d791facec91d37c72890995a0b48d31c70"
   integrity sha512-EqhiFU6daOA8kpjOWTL0olhVOF3i7OrFzSYiGsEMB8GcXS+RrzauAERX65xMeNWVqxA6HXH2m69Z9LaKKdisfg==