Update to gold service level

[stacey-budd]

Description

Simon Lucas asked for an update to the /support/ content to reflect the NHS Notify live support team's move from a silver to gold service level.

https://digital.nhs.uk/services/reference-guide#service-levels

Context

Following a discussion with Richard McStay, Simon and Richard agreed that the only change currently required was to amend the existing content to "gold service":

NHS Notify is a silver service (opens in a new tab).

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.

[emmagifford]

Happy to approve - links work, content looks good!