Use `CI_OPEN_MERGE_REQUESTS` environment variable

What does this MR do?

Use CI_OPEN_MERGE_REQUESTS environment variable

• This changes ci_pipeline_open_merge_requests to be on by default

• Related to #201845 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides