Update pages/404/index.js to not use CardHeaders new API

[tlabaj]

Is this a new page or a change to an existing one?
Change to 404 page

Describe the enhancement
The Card component's CardHeader API has changed. CardHeaderMain is no longer exported it is rendered internally.

This changes are in PR patternfly/patternfly-react#8759

[nicolethoen]

closed by #3429