Fix deprecation warnings in Praefect

From make verify-warnings

internal/praefect/conn/client_connections.go:37:32: grpc.CallCustomCodec is deprecated: use ForceCodec instead.  (SA1019)
internal/praefect/grpc-proxy/proxy/helper_test.go:42:4: grpc.CallCustomCodec is deprecated: use ForceCodec instead.  (SA1019)
internal/praefect/helper_test.go:281:32: grpc.CallCustomCodec is deprecated: use ForceCodec instead.  (SA1019)