Rename Workhorse terminal_specification to channel_specification

What does this MR do?

We're moving conceptually, from the term terminals to channels, in the context of creating WebSocket requests to Workhorse. In this MR we address that replacement.

Related MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9723

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Tests added for this feature/bug