NPM registry: replace the saj parser (17.1 backport)

🔭 Context

This is the %17.1 backport of !158978 (merged).

See that MR for more context details.

The related issue is NPM upload: the saj parser can pick up the wron... (#471979 - closed).

🤔 What does this MR do and why?

• Switch NPM package.json parsing from oj saj parsing to Gitlab::Json.parse.
• Update the related specs

🏎 MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).