Rename runner model's active flag to paused

The boolean active flag from the CiRunner model is not as clearly named as it could be. We should invert its meaning to paused so that its intent is clearer. This involves the following changes:

• Changing the database model (we may want to skip this due to the migration that it involves)
• GraphQL:
  • Deprecating the active field in the CiRunner GraphQL model (!79017 (merged));
  • Deprecating the active field in the RunnerUpdateInput GraphQL structure (!79131 (merged));
  • Deprecating the active filter in the Query.runners GraphQL query (!79018 (merged));
  • Adding a paused field to the CiRunner GraphQL model (!79017 (merged));
  • Adding a paused field in the RunnerUpdateInput GraphQL structure (!79131 (merged));
  • Adding a paused filter to the Query.runners GraphQL query (!79018 (merged));
• REST:
  • Add a paused field to the API::Ci::Runner :runners GET resource and :runner POST resource;
  • Deprecate the active field in endpoits returning Runner entities, such as:
    • GET /runners (!79244 (merged)/!79349 (merged))
    • POST /runners (!79244 (merged))
    • GET /runners/all (!79244 (merged)/!79349 (merged))
    • GET /runners/:id (!79244 (merged)) / PUT /runners/:id (!79244 (merged))
    • PUT --form "active=false" /runners/:runner_id (!79244 (merged))
    • GET /projects/:id/runners (!79244 (merged)/!79349 (merged)) / POST /projects/:id/runners (!79244 (merged))
    • GET /groups/:id/runners (!79244 (merged)/!79349 (merged))
  • gitlab-runner!3414 (merged) - Adapt Runner to use paused argument instead of active for registration with the PUT /runners/:id endpoint (gitlab-runner#28844 (closed) - only in %15.0 to avoid backward compatibility issues).

Deprecation issue: GraphQL API: Runner "active" nomenclature chang... (#351109 - closed)