Fix gRPC message size error in UpdateInvoices call

[andrew-farries]

The usage reconciler is currently failing with this error when invoking the UpdateInvoices RPC on the billing service:

{"@type":"type.googleapis.com/google.devtools.clouderrorreporting.v1beta1.ReportedErrorEvent","error":"failed to update invoices: rpc error: code = ResourceExhausted desc = grpc: received message larger than max (33395302 vs. 16777216)","level":"error","message":"Reconciliation run failed.","serviceContext":{"service":"usage","version":"commit-8e227e13ef2caae8f56c26387a78546b9fa93b30"},"severity":"ERROR","time":"2022-08-09T11:33:09Z"}