add deprecation warning for `allow_superuser: true`

[kaisecheng]

Release notes

Add deprecation warning to allow_superuser: true. Starting from version 9, this will be disabled by default. To avoid disruption during the upgrade, set 'allow_superuser' to true now if you wish to continue running as superuser temporarily after the upgrade.

What does this PR do?

• add deprecation warning when the current user id is 0 and allow_superuser is unset
• refactor existing deprecation log

Why is it important/What is the impact to the user?

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• relates: https://github.com/elastic/logstash/issues/16355

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