Removed z-index from flash container

What does this MR do?

Fixes a styling bug where the flash container would be above the header when scrolling.

Before it looked like this when scrolling:

Now it looks like this when scrolling:

Are there points in the code the reviewer needs to double check?

I can't think of any good reason why this would need to be above another element. But you never know!

Closes #20713 (closed)