PF4 Alert: Alert should default to "default" instead of "info"

[tlabaj]

1. The Alert should default to "default" instead of "info" alert
2. The default variant should be removed from the variant union since it has no matching modifier in core. Instead if the consumer does not provide variant type, the default (meaning no modifier) alert will be used.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[tlabaj]

closed by #3903