Set Environment Tier via API

Release notes

Previously, the only way to set the deployment_tier for an Environment was to use the keyword in the .gitlab-ci.yml file. In 15.0, we have added an endpoint to the Environment API to set the tier.

Problem

There is a feature Deployment tier of environments. Currently, users can set the tier via deployment_tier keyword, however, there is no way to set the tier via API.

Proposal

Add tier to Edit an existing environment API.