Replace PatientAccess COVID-19 advice with official NHS advice

Why is this change being made?

PatientAccess is a privately-owned application which links to non-NHS provided services which often incur a fee. It's also not available for all UK patients including many outside of England.

This MR updates GitLab's advice to UK residents in line with the UK government COVID-19 advice to use the NHS 111 service and the official NHS app. It also makes it more inclusive to those living outside England as coronavirus advice to the general public is set by devolved by administrations.

Author Checklist

• Correct MR template applied (e.g blog post)
• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

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