Replace Alert component "other" as default style

[aduth]

🛠 Summary of changes

Updates AlertComponent to replace the explicit :other variant to be used as the default, instead of :info.

For additional context, refer to 18F/identity-design-system#449

Stylistically, there is visual difference in the previous type: :other compared with the new default appearance, since we never actually applied the usa-alert--other class. This works to our benefit, since there's effectively no change as a result of the upcoming design system release, and no logic changes needed to remove the application of a usa-alert--other class.

As part of these changes, I searched code for existing usage of AlertComponent not specifying type with the assumption of rendering an :info alert banner and passed an explicit type: :info to avoid regressions.

📜 Testing Plan

Verify there are no regressions in the impacted code previously assuming default :info alert type.

Verify the "Other" type styles are now shown as the "Default" (type: nil) styles in the component preview:

1. Go to https://review-aduth-aler-cgle14.review-app.identitysandbox.gov/components/inspect/alert/preview (local: http://localhost:3000/components/inspect/alert/preview)

👀 Screenshots

Before	After