Cherry-pick to 7.x: [Filebeat] Ensure Kibana audit event.category and event.type are still processed as strings. (#25101)

[lukeelmers]

Backports the following commits to 7.x:

• [Filebeat] Ensure Kibana audit event.category and event.type are still processed as strings. ([Filebeat] Ensure Kibana audit event.category and event.type are still processed as strings. #25101)

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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• Build Cause: Pull request #25185 opened

• Start Time: 2021-04-20T21:14:27.010+0000

• Duration: 56 min 16 sec

• Commit: 1e5e202

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Test	Results
Failed	0
Passed	13577
Skipped	2299
Total	15876

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Test stats 🧪

Test	Results
Failed	0
Passed	13577
Skipped	2299
Total	15876