Disable NPM version 7

[fluiddot]

gutenberg PR: WordPress/gutenberg#30189

The Gutenberg project doesn't support NPM version 7 so we should disable this version in the Gutenberg-mobile too.

Two changes have been applied in this PR:

1. Match npm engine value in the package.json file with Gutenberg's project one.
2. Update the install node step in the CircleCI workflow so it doesn't enforce to install the latest NPM version.

To test:

1. Verify that the PR checks pass.
2. Verify that after doing the set up section in the README file, the NPM version installed is below v7.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[guarani]

Works for me, thanks @fluiddot!