Upgrade npm dependency

[ivankruchkoff]

Feature Description

Our package-lock.json lockfileVersion is at version 1, ( see https://raw.githubusercontent.com/google/site-kit-wp/develop/package-lock.json "lockfileVersion": 1, ).

Our .nvmrc specifies node 14 but doesn't specify a version of npm. NPM 6 is bundled with the release, but there's nothing stopping you from doing npm i -g npm@7

NPM v7 has an updated package lockfile version https://docs.npmjs.com/cli/v7/configuring-npm/package-lock-json/#lockfileversion

Running different versions of npm (even if it's the same version of node) creates big changes with every npm install.

We can solve this by doing one of the following:

• specify an npm engine version via https://docs.npmjs.com/cli/v6/configuring-npm/package-json#engines but this isn't strict and per the docs Unless the user has set the engine-strict config flag, this field is advisory only and will only produce warnings when your package is installed as a dependency.

We can do one of the following to standardise on npm7:

1. Introduce npm7 via config and also make it a point to upgrade our package-lock and reject CRs with lockfile version 1. OR
2. Bump node version to node 15 which includes npm7. This seems like the easier/more consistent option.

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Changelog entry

[aaemnnosttv]

Closing in favor of #6026