Increase Flash notification z-index to make it on top of the other elements

What does this MR do?

Increases sticky Flash notification z-index value to 251. It was 200 before but MR sticky tabs have z-index: 250.

Screenshots

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

Closes #31821 (closed)