Rename AlertsFinder option include_confidential to include_nonpublished

What does this MR do?

Context: We want to add a quick action which publishes an issue to a project's associated status page. Right now, the default is to publish all non-confidential issues in the project. With the quick action, issues will only be published if the quick action is used (and the issue is not confidential). A few minor naming tweaks will help prepare for this change without changing any logic.

This MR:

• Renames the include_confidential option for the AlertManagement::AlertsFinder to include_nonpublished

Why: This is going to make it easier to add an additional qualifier for whether an issue is published in the next MR.

Dependent feature MR: !30906 (merged) Related issue: #209849 (closed)

Logic changes are not included in this MR - no changelog will be required.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team