set `allow_superuser` to false as default

[kaisecheng]

Release notes

Running as superuser is disabled by default. To continue running Logstash as superuser temporarily after the upgrade, set 'allow_superuser' to true.

What does this PR do?

• set allow_superuser as false by default for v9
• change the buildkite image of ruby unit test to non-root

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
• [x] I have made corresponding changes to the documentation
• [x] 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

Use cases

Screenshots

Logs

[elastic-sonarqube[bot]]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: 89f3f12c31dcf80951b19cbdf1e7bc4473584218

History

• :green_heart: Build #1686 succeeded edb931fc9bd3b374ca5b9affe4c5a7b46688baa5
• :broken_heart: Build #1677 failed 0201bfc760a51dedb2e4e00acfff67f698bb3fc9

[TheVastyDeep]

@jsvd @kaisecheng You need to update the comment in logstash.yml that says the default is true.