Update Iteration Burnup/down Charts Version Intro

What does this MR do?

Updates the doc page for iteration burndown and burnup charts (https://docs.gitlab.com/ee/user/group/iterations/#iteration-burndown-and-burnup-charts) to reflect that this feature didn't make the 13.5 milestone and instead was introduced in 13.6. Related to this Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/180630

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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