Adapt Runner to use paused argument instead of active for registration

The active argument of the POST /api/v4/runners REST API endpoint was deprecated in gitlab!79137 (closed) (%14.8) in favor of the paused nomenclature. In order for the Runner to be able to register with %16.0+ GitLab instances, it needs to start sending the new paused argument.

Links

• Rename runner model's active flag to paused (gitlab#347211 - closed)
• REST API: Deprecate active field in POST /runners (gitlab!79137 - closed)