Remove unresolved class and fixed height in discussion header

What does this MR do?

This MR uses flex box to center the text content within the discussion header note wrapper rather than padding. Since the text is automatically centered, there is no longer a need for the unresolved CSS class, or the corresponding code that determines when to apply it.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Performance 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