Bump markdown from 3.7 to 3.8.1 #2501

Job	Run time
build-docker (webserver, docker/webserver.Dockerfile)	55s
build-docker (grafana, docker/grafana.Dockerfile)	50s
build-docker (linttest, docker/Dockerfile, linttest)	2m 47s
build-docker (test-runner, docker/test-runner.Dockerfile)	2m 46s
build-docker (unbound, docker/Dockerfile, unbound)	2m 47s
build-docker (util, docker/util.Dockerfile)	3m 57s
build-docker (internet.nl, docker/Dockerfile, app)	4m 41s
integration-test	7m 35s
lint/check	38s
development-environment-test	1m 56s
docs	3s
batch-integration-test	7m 28s
test	1m 58s
	38m 21s

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