docs: update npm-prune description

[Eomm]

The documentation uses a deprecated --production flag.
Using it shows the following warning:

$ npm prune --production
npm WARN config production Use `--omit=dev` instead.

The --production flag is not documented as well:

References

Relates to:

• Request: Remove the Warning about --production #6698

This PR makes clear that --production should not be used anymore.

[wraithgar]

Is there a reason this is on the v9 branch?

[Eomm]

I was using Node.js 18
Should I target latest and then it will be back-ported?

Did not found any guidance about this topic in https://github.com/npm/cli/blob/latest/CONTRIBUTING.md

[wraithgar]

We're still working out our official backporting policy, sorry. For now you should target latest and then once it's accepted you can make a second PR for the backport.

[wraithgar]

Thanks for both!