cisco_duo: fix invalid values for event.outcome

[efd6]

What does this PR do?

This fixes an invalid value in event.outcome for the auth data set.

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.

Author's Checklist

• [ ]

How to test this PR locally

See #3016 (comment)

Related issues

• Closes cisco_ftd, cisco_duo using invalid values according to ECS #3328

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• Start Time: 2022-05-11T13:20:50.703+0000

• Duration: 122 min 6 sec

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (5/5)	💚
Files	100.0% (5/5)	💚 3.589
Classes	100.0% (5/5)	💚 3.589
Methods	100.0% (50/50)	💚 11.716
Lines	91.506% (571/624)	👍 2.478
Conditionals	100.0% (0/0)	💚