[cisco_duo] Add event.created mapping

[andrewkroh]

What does this PR do?

Add missing mapping for event.created.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Start Time: 2022-04-07T00:40:18.478+0000

• Duration: 16 min 51 sec

Test stats 🧪

Test	Results
Failed	0
Passed	54
Skipped	0
Total	54

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