Flag the importance of updating old config files

[PhaedrusTheGreek]

We should add a note about how to handle old versions of config files. Even when using RPM, if defaults are changed, the RPM will not replace the old file, even in a major version upgrade, which can cause issues such as incorrect audit logging appender, incorrect JVM GC collector, etc.

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[elasticmachine]

Pinging @elastic/es-docs (>docs)