Use RollingUpdate strategy also for legacy proxy (#3814)

## Describe your changes
We have previously changed the neon-proxy to use RollingUpdate. This
should be enabled in legacy proxy too in order to avoid breaking
connections for the clients and allow for example backups to run even
during deployment. (https://github.com/neondatabase/neon/pull/3683)

## Issue ticket number and link
https://github.com/neondatabase/neon/issues/3333

.github/helm-values/prod-us-west-2-eta.neon-proxy-scram-legacy.yaml

@@ -1,6 +1,22 @@
 # Helm chart values for neon-proxy-scram.
 # This is a YAML-formatted file.
 
+deploymentStrategy:
+  type: RollingUpdate
+  rollingUpdate:
+    maxSurge: 100%
+    maxUnavailable: 50%
+
+# Delay the kill signal by 7 days (7 * 24 * 60 * 60)
+# The pod(s) will stay in Terminating, keeps the existing connections
+# but doesn't receive new ones
+containerLifecycle:
+  preStop:
+    exec:
+      command: ["/bin/sh", "-c", "sleep 604800"]
+terminationGracePeriodSeconds: 604800
+
+
 image:
   repository: neondatabase/neon