Externalize date picker string

What does this MR do?

I noticed that Date picker was not externalised in the CE, but apparently in EE. In my previous i18n MR !24514 (merged) this was the other way round.

Also, because the label is duplicated in CE, I'm not entirely sure whether externalising both copies is correct. Since date_picker.vue imports from ../pikaday.vue would it be useful to define & externalise the string only in one of the files?

I committed under the premise that it's probably safer to externalise both copies, rather than the wrong one.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9304

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer