Update to node 16 and npm 8 in the getting started with code contribution doc

[jblz]

What?

Correct an outdated reference to node and npm versions in the "Getting Started with Code Contribution" doc

Why?

Bring docs in line with the versions we're using:

gutenberg/package.json

Lines 17 to 20 in bc94b20

	"engines": {
	"node": ">=16.0.0",
	"npm": ">=8 <9"
	},

gutenberg/.nvmrc

Line 1 in bc94b20

16

How?

simple doc update

Testing Instructions

n/a

Testing Instructions for Keyboard

n/a

Screenshots or screencast

n/a

[github-actions]

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[alexstine]

👍