added new styling to the `.flash-warning` styles and moved the bundled style...

What does this MR do?

rebrush of the .flash-warning style definitions to make it look more like the design specs for it.

Solves #54169 (closed)

This is only changing the colors for now since all flash elements should be adjusted to adhere to the new design spec in another MR.

before

after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides