x/playground: redirect remaining playground urls to go.dev/play

https://play.golang.org/ already redirects to the new [go.dev/play](http://go.dev/play) UI.

Redirect goprevplay.golang.org/ to https://go.dev/play/?v=goprev
Redirect gotipplay.golang.org/ to https://go.dev/play/?v=gotip

For less confusion.

Start by locating the redirect configuration or entry point for goprevplay.golang.org and gotipplay.golang.org. Update the two hosts to redirect to their specified go.dev/play URLs, then verify both redirect targets and preserve the existing play.golang.org behavior.

Written by the indexing model from the issue text.