refactor: normalize package managers & CircleCI

[axe312ger]

Our 3 core packages, contentful.js, contentful-management.js and contentful-sdk-core all differ in the way they store and lock dependencies.

This is one of 3 PRs that will align that behavior:

• Migrate from yarn to npm
• Ensure lock file is committed to repository, but not included in package releases
• Ensure CircleCI uses modern, recommended way by CircleCI to install packages
• Ensure CircleCI uses the latest minor version of our minimum node version

Related PRs:

• https://github.com/contentful/contentful.js/pull/2164
• https://github.com/contentful/contentful-management.js/pull/2186

[contentful-automation[bot]]

:tada: This PR is included in version 8.1.3 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: