Move deployment secrets to github "environments"

[spbolton]

Ensure that secrets that should be specific to "deployments" tagged with a github entironment e.g. trunk, release are added to the environment configuration in Github, these secrets should be removed from the repository secrets. We should remove any repository secrets we no longer need.

As we cannot see the secret value to copy it, this task requires a discussion on who should create the secret and how it should be managed and rotated. Where possible we should assign different permissions in release vs trunk.

This task should document the secrets we need and information on expiry, permissions, and ownership of secrets.