NPA-6615: bump OAS version to 1.20.0 for release

[tomknapp]

Pull Request

🧾 Ticket Link

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

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📄 Description/Summary of Changes

• Spec version bump for the release

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🧪 Developer Testing Carried Out

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📋 PR Principles

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🏷️ Naming Conventions Reminder

Please ensure the following naming conventions are followed:

• PR title follows the format: NPA-XXXX: <short-description>
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[github-actions]

This branch is work on a ticket in the NHS Digital NPA JIRA Project. Here's a handy link to the ticket:

NPA-6615

[github-actions]

This branch is work on a ticket in the NHS Digital NPA JIRA Project. Here's a handy link to the ticket:

NPA-6615