chore: downgrade openapi to 3.0.0

[slowbackspace]

TODO:

• [x] no breaking change in TS types
• [ ] changes in fastify/json schema do not break backend https://github.com/blockfrost/openapi/pull/315/files#diff-33f9cc05a5c63837ad322d91bb968613732232017b2c96c136e7ce667b0207d8

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Name	Status	Preview	Comments	Updated (UTC)
openapi	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 30, 2023 3:22pm