Change `login_as` uses to `gitlab_sign_in`

spec/features/abuse_report_spec.rb

spec/features/admin/admin_abuse_reports_spec.rb

spec/features/admin/admin_active_tab_spec.rb

spec/features/admin/admin_appearance_spec.rb

spec/features/admin/admin_broadcast_messages_spec.rb

spec/features/admin/admin_browse_spam_logs_spec.rb

spec/features/admin/admin_browses_logs_spec.rb

spec/features/admin/admin_builds_spec.rb

spec/features/admin/admin_cohorts_spec.rb

...ures/admin/admin_conversational_development_index_spec.rb

spec/features/admin/admin_deploy_keys_spec.rb

...features/admin/admin_disables_git_access_protocol_spec.rb

spec/features/admin/admin_disables_two_factor_spec.rb

spec/features/admin/admin_groups_spec.rb

spec/features/admin/admin_health_check_spec.rb

spec/features/admin/admin_hook_logs_spec.rb

spec/features/admin/admin_hooks_spec.rb

spec/features/admin/admin_labels_spec.rb

spec/features/admin/admin_manage_applications_spec.rb

spec/features/admin/admin_projects_spec.rb