Update _envoy_istio.md (#8888)

Added a comment about allowing the http authorization headers to upstream, necessary in an istio meshConfig if there are proxy providers which inject http basic auth headers.

Signed-off-by: Wessel Valkenburg (prevue.ch) <116259817+valkenburg-prevue-ch@users.noreply.github.com>

website/docs/providers/proxy/_envoy_istio.md

@@ -20,6 +20,9 @@ spec:
                   headersToUpstreamOnAllow:
                       - set-cookie
                       - x-authentik-*
+                      # Add authorization headers to the allow list if you need proxy providers which
+                      # send a custom HTTP-Basic Authentication header based on values from authentik
+                      # - authorization
                   includeRequestHeadersInCheck:
                       - cookie
 ```