Update work item dates widget to use localized format

What does this MR do and why?

This MR updates the work item dates widget to use a localized format, to adhere to our guidelines https://docs.gitlab.com/ee/development/fe_guide/date_and_time.html

Changelog: fixed

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Change browser locale to something like en-GB
2. Go to an epic or issue and check the Dates widget is in en-GB format

MR acceptance checklist

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