Use HTML <time> elements for dates in changelog

[myakura]

Summary

Use semantic <time> to enhance machine readability.

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[GregBrimble]

Great idea, thank you!

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