Remove npm engine upper bound to fix EBADENGINE warning when installing web-ext using npm v7

[rpl]

Due to incompatibilities between the package lock file format generated by npm v7, any CI jobs running for nodejs version that are including npm v6 as the pre-installed npm version where failing as soon as renovatebot did upgrade the lockfile to lockfileVersion 2.

As a short term workaround, we added an upper bound on the npm engine in the wbe-ext package.json file (see ##2145).

With the npm upper bound set, installing web-ext using npm v7 will print an EBADENGINE warning:

npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: 'web-ext@6.1.0',
npm WARN EBADENGINE   required: { node: '>=12.0.0', npm: '>=6.9.0 <7.0.0' },
npm WARN EBADENGINE   current: { ... }
npm WARN EBADENGINE }

It doesn't prevent web-ext from being installed successfully, but it may be confusing for the users (e.g. because it may give the impression that web-ext isn't compatible with npm v7, which is not really the case, the package-lock.json does not matter while installing the package released on npm and it is not even included in the assets released on npm).

We should workaround the CI job issue as part of the circleci config and remove the upper bound from the npm engine configured in the web-ext package.json.

[rpl]

Closed by #2146