Follow-up for neondatabase/neon#2448 (#2452)

* remove `legacy` mode from the proxy readme
* explicitly specify `authBackend` in the link auth proxy helm-values
  for all envs

.github/helm-values/neon-stress.proxy.yaml

@@ -1,6 +1,7 @@
 fullnameOverride: "neon-stress-proxy"
 
 settings:
+  authBackend: "link"
   authEndpoint: "https://console.dev.neon.tech/authenticate_proxy_request/"
   uri: "https://console.dev.neon.tech/psql_session/"
 

.github/helm-values/production.proxy.yaml

@@ -1,4 +1,5 @@
 settings:
+  authBackend: "link"
   authEndpoint: "https://console.neon.tech/authenticate_proxy_request/"
   uri: "https://console.neon.tech/psql_session/"
 

.github/helm-values/staging.proxy.yaml

@@ -5,6 +5,7 @@ image:
   repository: neondatabase/neon
 
 settings:
+  authBackend: "link"
   authEndpoint: "https://console.stage.neon.tech/authenticate_proxy_request/"
   uri: "https://console.stage.neon.tech/psql_session/"