Update cloudfunctions runtimes and show nodejs16

[modular-magician]

This change updates cloudfunctions examples to use the currently recommended nodejs runtime, and revises the list of example runtimes to those currently recommended in the product documentation.

Not sure if there's a need to run the various builds and tests for a markdown change?

This is similar to GoogleCloudPlatform/magic-modules#4775

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Derived from GoogleCloudPlatform/magic-modules#6029

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