[EuiSuperDatePicker] Date formatting and i18n

[thompsongl]

The formatTimeString util will return string values that bypass i18n (now, Invalid Date).
Investigate how best to incorporate i18n.

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I'd like to see this use i18n, but none of the other values from this function use it and it's unclear how we'd adjust the output.

I'll open a separate issue to explore; this doesn't need to be a blocker for this PR.

Originally posted by @thompsongl in #5263 (comment)

[github-actions]

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[thompsongl]

Resolved by #5743