feat(v2): support/use React v17 by default (#4218)

* chore(v2): allow React v17 as peer dependency

* minor React 17 changes

* Revert examples

* Possible fix

* Use @docusaurus/react-loadable fork

* add back react-loadable to please eslint/ts/linters, even if it's unused in practice

* missing dependency bump

* move enzyme dependencies to docusaurus v1

Co-authored-by: slorber <lorber.sebastien@gmail.com>

examples/classic/package.json

@@ -33,4 +33,4 @@
     ]
   },
   "description": "Docusaurus example project"
-}
+}

examples/facebook/package.json

@@ -50,4 +50,4 @@
     ]
   },
   "description": "Docusaurus example project (facebook template)"
-}
+}