[Docs] Document log4j2.properties default rolling strategy

[edmocosta]

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Description:

We should document the default rolling strategy configured on Logstash log4j2.properties. It's also important to explain that it does not provide any default archiving action and that logs are store indefinitely, with a maximum of 30 files per day, following the time and size policies.

Example of archiving strategy for cleaning logs older than 30D:

appender.rolling.strategy.action.type = Delete
appender.rolling.strategy.action.basepath = ${sys:ls.logs}
appender.rolling.strategy.action.maxdepth = 1
appender.rolling.strategy.action.condition.type = IfLastModified
appender.rolling.strategy.action.condition.age = 30D
appender.rolling.strategy.action.pathConditions.type = IfFileName
appender.rolling.strategy.action.pathConditions.glob = logstash-plain-*.log.gz

URL:

https://www.elastic.co/guide/en/logstash/current/logging.html

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Another option would be deciding about the #7482 issue.