Add delete environment job to chores

Description

This adds the details about the new job that deletes old environments in the TW chores tasks. I tested it today and it appears to work well:

• Pipeline: https://gitlab.com/gitlab-org/gitlab-docs/-/pipelines/569789894
• Job: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/2622764985

Related issues

• Related to gitlab-docs#1205 (closed)
• Related to gitlab-docs!2847 (merged)