v1.0.556-alpha

NPA-6615: bump OAS version to 1.20.0 for release (#338)

# Pull Request

## 🧾 Ticket Link

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https://nhsd-jira.digital.nhs.uk/browse/NPA-6615

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## 📄 Description/Summary of Changes

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purpose/scope/impact of the changes -->

- Spec version bump for the release

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## 🧪 Developer Testing Carried Out

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## 🏷️ Naming Conventions Reminder

Please ensure the following naming conventions are followed:

- PR title follows the format: `NPA-XXXX: <short-description>`
- Branch name follows the convention:
`<type>/NPA-XXXX/<short-description>`
- Commit messages follow the template: `NPA-XXXX: <short-description>`