[7.13](backport #25609) [Filebeat] Fix checkpoint.action_reason when its a string, not a Long

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This is an automatic backport of pull request #25609 done by Mergify.
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Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   filebeat/docs/fields.asciidoc
	modified:   x-pack/filebeat/module/checkpoint/firewall/_meta/fields.yml
	modified:   x-pack/filebeat/module/checkpoint/firewall/ingest/pipeline.yml
	modified:   x-pack/filebeat/module/checkpoint/firewall/test/checkpoint_with_time.log
	modified:   x-pack/filebeat/module/checkpoint/firewall/test/checkpoint_with_time.log-expected.json

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/filebeat/module/checkpoint/fields.go

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• Build Cause: P1llus commented: /test

• Start Time: 2021-05-10T15:23:46.091+0000

• Duration: 31 min 55 sec

• Commit: 88c0443

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Skipped	188
Total	1116

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Total	1116

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/test