NPA-6718: bump OAS version to 1.21.0 for release

[davesmallnhs]

Pull Request

🧾 Ticket Link

https://nhsd-jira.digital.nhs.uk/browse/NPA-6718

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📄 Description/Summary of Changes

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🧪 Developer Testing Carried Out

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📋 PR Principles

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Please ensure the following naming conventions are followed:

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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-6718