wip: Add severity level

[gaelgoth]

Checklist:

• [ ] I have updated the necessary documentation
• [ ] I have signed off all my commits as required by DCO
• [ ] My build is green

The purpose of this draft is to add the notion of announcements criticality level. I think this data has the same lifecycle as an announcement, meaning that I plan to add a new column level in announcements table.

This new entry will allow us to implement this timeline: https://github.com/procore-oss/backstage-plugin-announcements/issues/119 and reintroduce a different color banner based on the level.

https://github.com/procore-oss/backstage-plugin-announcements/assets/55390114/c3a40b3f-99d8-4710-8cf1-0635fd3dc039

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