Fix Mend kev_date format - add conversion

[testaccount90009]

Description

Sorry for not catching this in the last PR - the kev_date (publishDate) is not as I expected from the Mend API. Fixing this by adding a conversion.

Checklist

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• If this is a new feature and not a bug fix, you've included the proper documentation in the docs at https://github.com/DefectDojo/django-DefectDojo/tree/dev/docs as part of this PR.
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• Add applicable tests to the unit tests.
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Contributors: Git Tips

Rebase on dev branch

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Squashing commits

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[dryrunsecurity]

No security concerns detected in this pull request.

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[mtesauro]

Approved