Always use en-us format for deprecations dates

What does this MR do and why?

Deprecations dates are using the browser default language for date formatting. This is confusing when everything else is in English.

While we will eventually need these localized, for now everything should just be in English (US).

Reported by @Taucher2003 on Slack: https://gitlab.slack.com/archives/C16HYA2P5/p1715151004908479

Screenshots, screen recordings, or links to review app

before	after

Review app: https://deprecations-month-delocalize.docs.gitlab-review.app/ee/update/deprecations.html#behavior-change-for-protected-variables-and-multi-project-pipelines

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: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Adjust your OS settings to use a different language than English, like German (likely requires restarting your laptop)
3. View the deprecations page on the live site; the dates are localized: https://docs.gitlab.com/ee/update/deprecations.html#behavior-change-for-protected-variables-and-multi-project-pipelines
4. View the deprecations page locally; the dates should not be localized and should be in en-us format: http://localhost:3000/ee/update/deprecations.html#behavior-change-for-protected-variables-and-multi-project-pipelines

Merge request 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 merge request.