[Docs]: Warning logs contains link leading to a 404

[BenoitZugmeyer]

Describe what's incorrect/missing in the documentation

With v6.28.0, we started to have warnings like:

• ⚠️ React Router Future Flag Warning: React Router will begin wrapping state updates in React.startTransition in v7. You can use the v7_startTransition future flag to opt-in early. For more information, see https://reactrouter.com/v6/upgrading/future#v7_starttransition.
• ⚠️ React Router Future Flag Warning: Relative route resolution within Splat routes is changing in v7. You can use the v7_relativeSplatPath future flag to opt-in early. For more information, see https://reactrouter.com/v6/upgrading/future#v7_relativesplatpath.

Those links are leading to a 404 page. On my machine, https://reactrouter.com/v6/upgrading/future redirects to https://reactrouter.com/en/6.28.0/upgrading/future which is non-existant.

[timdorr]

Duplicate of #12383