chore: deprecate Gateway.PathPrefixes

Context:
ipfs/kubo#7702 (comment)


This commit was moved from ipfs/kubo@2f105f7

gateway/core/corehttp/gateway_handler.go

@@ -159,6 +159,7 @@ func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request
 	// If the gateway is behind a reverse proxy and mounted at a sub-path,
 	// the prefix header can be set to signal this sub-path.
 	// It will be prepended to links in directory listings and the index.html redirect.
+	// TODO: this feature is deprecated and will be removed  (https://github.com/ipfs/go-ipfs/issues/7702)
 	prefix := ""
 	if prfx := r.Header.Get("X-Ipfs-Gateway-Prefix"); len(prfx) > 0 {
 		for _, p := range i.config.PathPrefixes {