fix missing lastUpdated property in docs

[hanish-rishen]

Closes #7131 by adding the missing property description to the API documentation.

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Which Traffic Control components are affected by this PR?

• Documentation

What is the best way to verify this PR?

Go to the link below and verify the response description.

• https://traffic-control-cdn.readthedocs.io/en/latest/api/v3/staticdnsentries.html
• https://traffic-control-cdn.readthedocs.io/en/latest/api/v4/staticdnsentries.html
• https://traffic-control-cdn.readthedocs.io/en/latest/api/v3/staticdnsentries.html

If this is a bugfix, which Traffic Control versions contained the bug?

• historical/legacy/EoL versions
• 6.x
• 7.x
• master

PR submission checklist

• This PR needs no tests, it is a minor documentation change.
• This PR itself is a documentation fix.
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY