Fully migrate to new deployment model

[angela-tran]

• Create a main branch, add branch protection rules similar to that of dev
• Rename references to dev branch to use main
• Add logic to tag main branch's Docker image as dev
• Update pipeline logic for getting deployment environment to consider main branch
• Remove logic for deriving Docker tag name from Git branches for test/prod
• Delete test and prod branch triggers from azure-pipeline.yml and deploy.yml
• Make main the default branch in the repo

After close / merge of #414

• Delete dev, test, and prod branches and branch protection rules
• Update main branch protection to require the tests check

[thekaveman]

Created main branch from the state of dev as of the most recent full deploy/release 2024.01.12