Dockerfile: Bump grafana/xk6 from 1.3.7 to 1.4.0 in /tracker/test/load (#1366)

Author: dependabot[bot]

tracker/test/load/Dockerfile.k6

@@ -4,7 +4,7 @@
 # Build k6 with the xk6-mqtt extension for MQTT load testing.
 # xk6 Docker tag 1.3.7 bundles xk6 Go module v1.3.7.
 # See: https://github.com/grafana/xk6/releases/tag/v1.3.7
-FROM grafana/xk6:1.3.7@sha256:a5543f005b660458843738a1b571d490ac1ea3d2b9a58375fec8553b196ac011 AS builder
+FROM grafana/xk6:1.4.0@sha256:3a3d4b02be2ebffa693f98c6699071a082a6b47ab106c6ba1ac72e15f7550b88 AS builder
 ARG http_proxy
 ARG https_proxy
 ARG no_proxy