chore: stop calling deprecated grpcServer.start

[trentm]

As of @grpc/grpc-js 1.10.0 '.start()' is deprecated and a no-op.

Refs: #60
Refs: https://github.com/grpc/proposal/blob/master/L107-node-noop-start.md

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Before this change mockotlpserver would show a deprecation warning on startup:

% node --trace-deprecation lib/cli.js
{"name":"mockotlpserver","level":30,"msg":"OTLP/HTTP listening at http://127.0.0.1:4318/","time":"2024-02-12T17:33:35.493Z"}
{"name":"mockotlpserver","level":30,"msg":"OTLP/HTTP listening at http://localhost:4317/","time":"2024-02-12T17:33:35.500Z"}
(node:72369) DeprecationWarning: Calling start() is no longer necessary. It can be safely omitted.
    at /Users/trentm/el/elastic-otel-node2/packages/mockotlpserver/lib/grpc.js:97:42
    at /Users/trentm/el/elastic-otel-node2/node_modules/@grpc/grpc-js/build/src/server.js:567:25
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
{"name":"mockotlpserver","level":30,"msg":"UI listening at http://127.0.0.1:8080/","time":"2024-02-12T17:33:35.502Z"}