fix: use kebab-case for category deserialization

[d-loose]

I noticed that update_snap_categories wasn't working when running the ratings server locally:

{"timestamp":"2024-12-13T11:31:42.208055Z","level":"ERROR","message":"unable to update snap categories: Matching variant not found","snap_id":"3wdHCAVyZEmYsCMFDE9qt92UV8rC8Wdk","target":"ratings::ratings::categories","filename":"src/ratings/categories.rs","line_number":55,"threadId":"ThreadId(16)"}

The get_snap_categories_works test (which is skipped in CI since it's sending queries to snapcraft.io) was failing as well.