Use GitHub Actions for CI

[coldlink]

What does this change?

Switches CI from TeamCity to GitHub Actions (with the use of https://github.com/guardian/actions-assume-aws-role).

There are a few benefits to using GHA individually:

• No build queue
• No need to manage node docker versions
• No need to globally install yarn
• Build logs are co-located with the PR (via the Checks tab)
• Collectively, use of GHA means we don't have to maintain our own CI infrastructure.

In RiffRaff the build numbers will count up from 5000 now, as the teamcity builds are currently in the 4000s.

To help perform the migration, I used migration doc and example prs from Prism and tag-janitor.

How to test

identity:identity-gateway should appear as a deployable project in Riff-Raff. It should be successfully deployed to CODE.

https://riffraff.gutools.co.uk/deployment/view/efa79594-2d2f-4b0d-8339-3e591bf3873f

Pre merge

• Change the branch protection rules to remove TeamCity from required, and add the CI Github Action to required

Post merge

• Ask people to rebase their branches on main to deploy to CODE/PROD going forward
• Remove project from TeamCity