Change JSON key dist_tags to dist-tags in npm metadata cache

What does this MR do and why?

Part of #393655 (closed)

Align JSON key dist-tags in npm metadata cache with origin package metadata:
https://github.com/npm/registry/blob/master/docs/responses/package-metadata.md#abbreviated-metadata-format
to avoid any confusion between dist_tags and dist-tags.

Screenshots or screen recordings

No.

How to set up and validate locally

All steps need to be executed in rails console

1. Create a new npm package:

   def fixture_file_upload(*args, **kwargs)
     Rack::Test::UploadedFile.new(*args, **kwargs)
   end
   
   package = FactoryBot.create(:npm_package, project: Project.first)

2. Execute the service to create a new entity of Packages::Npm::MetadataCache:

   Packages::Npm::CreateMetadataCacheService.new(package.project, package.name).execute

3. Verify that the metadata JSON from a new entity of Packages::Npm::MetadataCache contains dist-tags key on the top level:

   metadata_cache = Packages::Npm::MetadataCache.last
   Gitlab::Json.parse(metadata_cache.file.read)

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #393655 (closed)