Fix rubocop for gitlab json

What does this MR do and why?

fix rubocop rule for Gitlab::Json

jh/app/controllers/jh/projects/graphs_controller.rb:34:22: C: [Correctable] Gitlab/Json: Prefer Gitlab::Json over calling JSON or to_json directly. See https://docs.gitlab.com/ee/development/json.html

        render json: @log.to_json
                     ^^^^^^^^^^^^
jh/lib/gitlab/content_validation/client.rb:16:34: C: [Correctable] Gitlab/Json: Prefer Gitlab::Json over calling JSON or to_json directly. See https://docs.gitlab.com/ee/development/json.html

        response = request(path, { content: content }.to_json)
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
jh/lib/gitlab/content_validation/client.rb:28:58: C: [Correctable] Gitlab/Json: Prefer Gitlab::Json over calling JSON or to_json directly. See https://docs.gitlab.com/ee/development/json.html

        request("/api/content_validation/blob_validate", data.to_json)
                                                         ^^^^^^^^^^^^
jh/lib/gitlab/content_validation/client.rb:34:54: C: [Correctable] Gitlab/Json: Prefer Gitlab::Json over calling JSON or to_json directly. See https://docs.gitlab.com/ee/development/json.html

        request("/api/content_validation/complaint", data.to_json)
                                                     ^^^^^^^^^^^^
jh/spec/support/helpers/jh/content_validation_request.rb:21:17: C: [Correctable] Gitlab/Json: Prefer Gitlab::Json over calling JSON or to_json directly. See https://docs.gitlab.com/ee/development/json.html

          body: { 'code' => 200, 'message' => 'validation success"' }.to_json,
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
jh/spec/support/helpers/jh/content_validation_request.rb:28:17: C: [Correctable] Gitlab/Json: Prefer Gitlab::Json over calling JSON or to_json directly. See https://docs.gitlab.com/ee/development/json.html

          body: { code: 406, message: "validation block" }.to_json,
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
jh/spec/support/helpers/jh/content_validation_request.rb:46:15: C: [Correctable] Gitlab/Json: Prefer Gitlab::Json over calling JSON or to_json directly. See https://docs.gitlab.com/ee/development/json.html

        body: { code: 406, message: "validation block" }.to_json,
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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