Update custom error pages repo

[poornima-krishnasamy]

Background

The custom error pages repo is used as a Cloud Platform default backend. The repo used to build the image is outdated and archived.

Approach

Update the repo code to use lated golang and build a automated deployment step.

https://github.com/ministryofjustice/cloud-platform-custom-error-pages
https://github.com/ministryofjustice/cloud-platform-terraform-ingress-controller/blob/main/variables.tf#L23

Which part of the user docs does this impact

#5149

Communicate changes

• post for #cloud-platform-update
• Weeknotes item
• Show the Thing/P&A All Hands/User CoP
• Announcements channel

Questions / Assumptions

Definition of done

• readme has been updated
• container image issue have been updated
• another team member has reviewed
• smoke tests are green
• prepare demo for the team

Reference

How to write good user stories

[poornima-krishnasamy]

@jackstockley89 , can you use the latest release of the custom-error-pages in ingress controller terraform module, test the default backend on test cluster and apply that to live-2, manager and live.