Enable ci_config_merged_tab feature by default

What does this MR do?

Enable ci_config_merged_tab feature flag by default and adds the changelog.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Style guides

Availability and Testing

Merged tab is contained on its own and should it breaks, it would mean that the content of the tab would be empty. There is a default error state that would catch this. So far on multiple projects, it works as intended.