Add support for NPM V6 deprecation warning and unsupported error

[amazimbe]

What are you trying to accomplish?

Add changes to set NPM v6 as deprecated and unsupported. The deprecation warning and unsupported error are controlled by the following feature flags:

• npm_v6_deprecation_warning
• npm_v6_unsupported_error

Feature Flags PR: https://github.com/github/dependabot-api/pull/5763

This mirrors the approach used for Bundler v1 and composer 1, allowing us to guide users towards migrating to NPM v9 while providing a controlled rollout.

Anything you want to highlight for special attention from reviewers?

The deprecation and unsupported checks for NPM v6 are controlled by feature flags. Please focus on verifying that the logic tied to the feature flags works as expected.

How will you know you've accomplished your goal?

We will monitor logs and metrics to ensure:

• NPM v6 users see the deprecation warning when the feature flag is enabled.
• Actions using NPM v6 are blocked when the unsupported error flag is enabled.
• Tests have been added to validate the behaviour of both feature flags.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[markhallen]

LGTM