[TI AbuseCH] Fix field mappings

[legoguy1000]

What does this PR do?

Fix field mapping conflicts in threat.indicator.file.x509.not_before/not_after

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

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How to test this PR locally

Related issues

• Closes [ti_abusech] Conflicting threat.indicator.file.x509.not_before/not_after definition #2871

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• Start Time: 2022-03-28T00:35:23.257+0000

• Duration: 14 min 12 sec

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Total	18

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