[Okta] Add missing ECS field mapping event.created

[P1llus]

What does this PR do?

Adds a missing field definition 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.

Related issues

• Closes [okta] Missing ECS created field #2698

[elasticmachine]

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

[elasticmachine]

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Build stats

• Start Time: 2022-03-07T16:12:05.356+0000

• Duration: 12 min 24 sec

Test stats 🧪

Test	Results
Failed	0
Passed	12
Skipped	0
Total	12

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