Disable allow_superuser by default

[robbavey]

In #14046 and #14089, we introduced new functionality to guard against running Logstash as root with the introduction of a new setting allow_superuser. The initial implementation set this to true to avoid a breaking change in a minor release, but we should set this setting to false by default to disallow running Logstash as root by default

Tasks

Beta Give feedback

1. on main (8.x), when allow_superuser is not set, log a deprecation advising how to resolve prior to upgrading to 9.x by either (1) setting allow_superuser: false to opt in or (2) setting allow_superuser: true to opt out
2. on 9.x, set default value of allow_superuser to false

[flexitrev]

Double check description here, was initial implementation set to true?

[robbavey]

Yes it was