Disable allow_superuser by default

[robbavey]

In https://github.com/elastic/logstash/pull/14046 and https://github.com/elastic/logstash/pull/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
- [ ] 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
- [ ] 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