[8.17](backport #6109) Make redaction of common keys in diagnostics case insensitive

[mergify]

What does this PR do?

Changes the redaction of common keys like "password" to be case insensitive. This means keys like "PASSWORD" and "PassWord" will also be redacted.

Why is it important?

To prevent leaking secrets into diagnostics.

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 couldn't find any docs on diagnostics redaction
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• [ ] I have added an integration test or an E2E test

Disruptive User Impact

This may cause keys that were previously not redacted to be redacted after the change. For example, the "Key" substring may be common in configuration keys that don't need redaction.

How to test this PR locally

cd internal/pkg/diagnostics
go test .
```<hr>This is an automatic backport of pull request #6109 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 40%)

See analysis details on SonarQube