Improve chart labels on Issues Analytics

What does this MR do?

Previously the chart on Issues Analytics referred to "issues created", which is developer terminology. This changes it to "issues opened", because "opened" is the term used throughout the UI.

Note: The documentation does not make reference to these labels, so does not need to change.

This was prompted by #196547 (comment 326336247)

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