Fixed a bug where shards could not resolve individual proxies. (#4188)

## Motivation

An omission of the correct `Service` name in the `serviceName`
definition of the proxy statefulset meant that proxy instances could not
be uniquely resolved by ordinal.

## Proposal

Update the `serviceName`.

## Test Plan

Tested manually.

## Future Work

As was suggested by @Twey, it might be better to not use ordinals, but
instead do DNS lookups to resolve individual IP addresses and use those
instead. Since individual proxy instances are undifferentiated, ordering
is unimportant (unlike the current shard setup for example).

Author: christos-h

kubernetes/linera-validator/templates/proxy.yaml

@@ -39,7 +39,7 @@ kind: StatefulSet
 metadata:
   name: proxy
 spec:
-  serviceName: "proxy"
+  serviceName: "proxy-internal"
   selector:
     matchLabels:
       app: proxy