Refactor concourse pipelines

[drewbo]

We'd like to refactor our concourse pipelines for greater consistency across our repos. We've set up pages-pipeline-tasks to assist with this. Roughly:

• Abstract common tasks into tasks/scripts
• Add common/repeated resources/resource types to the overlays here to be replaced by ytt
• Move all our repositories to having one long-running branch (main) and setup the pipelines so that PRs deploy to dev, merges to main deploy to staging, and new matching tags deploy to production
• Semi-automate the release process like in this repo: we have an auto-updating release branch which when merged creates a new tag/release/deploy to production
• Any other simplifications we see that can do across our pipelines now that container hardening is complete.

Resources:

• https://docs.google.com/spreadsheets/d/1WJqK-wKLrnxYZhd1RHq1w-t6RQu3a58JlU_UuW9NjcQ/edit#gid=0

  
  ### Tasks

• [ ] https://github.com/cloud-gov/pages-build-container/issues/472
• [ ] https://github.com/cloud-gov/pages-proxy/pull/212
• [ ] https://github.com/cloud-gov/pages-mailer/pull/43
• [ ] https://github.com/cloud-gov/pages-core/issues/4548
• [ ] https://github.com/cloud-gov/pages-redirects/issues/249
• [ ] https://github.com/cloud-gov/pages-cf-build-tasks/issues/37

[drewbo]

Still likely to tinker with a few pipelines after the fact but this is done!