Restrict npm version explicitly set in the package.json as supported to prevent renovatebot from updating the lock file to version 2

[rpl]

Some recent renovatebot PRs (#2141, #2142, #2143) are failing on CI while running the npm ci step.

The reason for the failure (as described in #2142 (comment)) seems to be triggered because the lockfileVersion: 2 that renovatebot did include in those PR is currently incompatible with npm v6.

As a short term solution we agreed to try to require in the package.json an npm version strictly < than npm v7, which should hopefully be enough to force renovatebot to don't upgrade the lockfileVersion yet.

We should then remove that restriction from the package.json once we are ready to fully migrate to npm v7 (or if in the meantime the incompatibility of the lockfileVersion: 2 with npm v6 is gone).