Deprecate Types::Ci::RunnerUsageType#ci_minutes_used and rename to ci_duration

CI Minutes is a loaded word, since we use that term for billing (to which we apply a cost factor). Since this GraphQL field doesn't have that factor applied, we should avoid calling it ci_minutes_used and instead rename it to something like ci_duration.

Usage data

In the last 7 days there has been no usage outside of the GitLab.com usage (MR removing usage of ciMinutesUsed):

https://log.gprd.gitlab.net/app/r/s/3gRJu