Fix changelog generation for the single codebase

What does this MR do?

Instead of checking the presence of CHANGELOG-EE.md to determine where changelog files should go, we rely on the -e/--ee flag to be provided. When not specified, changelog files go in ./changelogs. When it is specified, changelog files go in ./ee/changelogs.

Fixes #32040 (closed)

Does this MR meet the acceptance criteria?

Conformity

• 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