[slack] Parse action_timestamp as microseconds

[andrewkroh]

What does this PR do?

Parse action_timestamp as microseconds since unix epoch. In #4999 we had samples that showed the value as unix_ms so this only parses the value as microseconds when it is larger than 1e13.

Fixes errors like:

Cannot index event ... {"type":"document_parsing_exception","reason":"[1:994] failed to parse field [@timestamp] of type [date] in document with id 'cLmsfZpQNjWMIUJcO00CXAZ4jgM='. Preview of field's value: '+55128-03-07T16:27:34.646Z'","caused_by":{"type":"illegal_argument_exception","reason":"failed to parse date field [+55128-03-07T16:27:34.646Z] with format [strict_date_optional_time||epoch_millis]","caused_by":{"type":"date_time_parse_exception","reason":"Failed to parse with all enclosed parsers"}}}, dropping event!

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.

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• Start Time: 2023-07-14T19:31:29.847+0000

• Duration: 14 min 47 sec

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Failed	0
Passed	5
Skipped	0
Total	5

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚 6.25
Classes	100.0% (1/1)	💚 6.25
Methods	100.0% (14/14)	💚 14.286
Lines	98.715% (384/389)	👍 12.789
Conditionals	100.0% (0/0)	💚

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

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Package slack - 1.7.1 containing this change is available at https://epr.elastic.co/search?package=slack