modsecurity: fix handling of apache audit.details field

[efd6]

What does this PR do?

Apache provides these as an array of strings, but a flattened must be an object or array of objects. So promote the strings to an object with the string in the "value" field.

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

• Fixes [modsecurity] parsing exception - failed to parse field [modsec.audit.details] #6579

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• Start Time: 2023-06-22T09:58:08.608+0000

• Duration: 15 min 49 sec

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (3/3)	💚
Classes	100.0% (3/3)	💚
Methods	100.0% (30/30)	💚 3.846
Lines	91.582% (359/392)	👎 -8.418
Conditionals	100.0% (0/0)	💚

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Package modsecurity - 1.9.1 containing this change is available at https://epr.elastic.co/search?package=modsecurity