pin gov.uk frontend to 6.5.0

[JC-DSIT]

Proposed changes

What changed

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Related issue or tracking reference

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• Fixes #
• Relates to #

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Screenshots or examples (if relevant)

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Checklist

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• the pull request has a clear title with a short description about the update in the documentation
• GitHub actions all pass
• you have tested the changes with a fresh build against the latest version of the tech-docs-gem
• you have linked this PR to an issue (or explained why none exists)
• you have updated documentation if needed