Updated getting started guide to use Node v18.18.0

[kbennett2000]

Updated getting started guide to use Node v18.18.0 instead of LTS.

work-around for:

• https://github.com/Agoric/dapp-agoric-basics/issues/35

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[dckc]

Let's be sure the motivation is documented:

• https://github.com/Agoric/dapp-agoric-basics/issues/35