fix(PPDSC-2106): correct minimum supported node version

[mstuartf]

PPDSC-1206

What

1. Background: Some tests (e.g. src/instrumentation/tests/analytics-instrumentation.test.tsx) make assertions on error messages, which depend on the node version being used. We need to make sure the correct version for the tests to pass is specified in the package.json and README.
2. Set minimum supported node version in package.json and README to 16.9.1.

I have done:

• Written unit tests against changes
• Written functional tests against the component and/or NewsKit site
• Updated relevant documentation

I have tested manually:

• The feature's functionality is working as expected on Chrome, Firefox, Safari and Edge
• The screen reader reads and flows through the elements as expected.
• There are no new errors in the browser console coming from this PR.
• When visual test is not added, it renders correctly on different browsers and mobile viewports (Safari, Firefox, small mobile viewport, tablet)
• The Playground feature is working as expected

How to test:

• Switch to minimum supported node version and run yarn test locally .

[newskit-engineering]

You can preview these changes on:
Documentation Site: http://ncu-newskit-docs.s3-website-eu-west-1.amazonaws.com/ppdsc-2106-correct-minimum-node-version
Storybook: http://ncu-newskit-docs.s3-website-eu-west-1.amazonaws.com/ppdsc-2106-correct-minimum-node-version/storybook