Change :feature_category to :runner_fleet in specs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/106628



Merged-by: Andy Soiron <asoiron@gitlab.com>
Approved-by: Andy Soiron <asoiron@gitlab.com>
Reviewed-by: Andy Soiron <asoiron@gitlab.com>
Co-authored-by: Pedro Pombeiro <noreply@pedro.pombei.ro>