Adds a changelog for elastic stack feature

What does this MR do?

I mistakenlly suggested to remove a changelog entry, but apparently we should include a changelog even for flagged features if these have a DB migration.

Related to this discussion !18015 (comment 233699430)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• [-] 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