Hide versions menu for ja-JP

What does this MR do and why?

As we're not translating any of the older docs pages, we're hiding the versions menu drowdown in ja-JP. Here's the changes:

1. Modifies the themes/gitlab-docs/layouts/_partials/header.html file.
2. As a result:
   • If the selected language is en-US: all of the previous menu versions and archive link will be shown in the versions selector.
   • If the selected language is ja-JP: the versions menu will be hidden.

FYI - Once the system is mature enough, we may need to bring the menu back in ja-JP later on in the future. But that should be a simple one line change in this file: themes/gitlab-docs/layouts/_partials/header.html

Related: gitlab-com/localization/docs-site-localization#532 (closed)

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment.
2. Check out this branch.
3. Fire up in your local using make setup and make view
4. Change language selector to Japanese and check out the versions menu drop down to verify.

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