Prevent using npm 7 until gutenberg supports it

[jorbin]

Since ​https://github.com/WordPress/gutenberg/pull/29204, gutenberg has required NPM 6. As much of WordPress depends on JS from there, we should match NPM requirements

[rcorrales]

I attached a patch that limits the npm version to 6 and sets engine-strict = true in .npmrc so it stops the installation if the version is not supported.

[rcorrales]

You can test this fix by installing a version > 6:

npm i -g npm@7

Then, when you would get this error when running npm install:

npm ERR! code EBADENGINE
npm ERR! engine Unsupported engine
npm ERR! engine Not compatible with your version of node/npm: WordPress@6.1.0
npm ERR! notsup Not compatible with your version of node/npm: WordPress@6.1.0
npm ERR! notsup Required: {"node":">=14.15.0","npm":">=6.14.8 <7"}
npm ERR! notsup Actual:   {"npm":"7.24.2","node":"v16.13.0"}

Testing https://core.trac.wordpress.org/ticket/56547

[jorbin]

In 54124:

Build/Test: Prevent using unsupported NPM versions.

Using NPM 7+ currently causes a number of issues. In order to improve developer experience, we should prevent a developer from going down the road of using incompatible engines.

See also: ​https://github.com/WordPress/gutenberg/pull/29204 and ​https://github.com/WordPress/gutenberg/pull/23600.

Props rcorrales.
Fixes #56547.

Merged in https://core.trac.wordpress.org/changeset/54124