Systemd logs warnings about the legacy tmpfile location /var/run

[lukebakken]

https://access.redhat.com/solutions/4154291

Reported here: https://groups.google.com/g/rabbitmq-users/c/uCrWpsTzSWA

References to /var/run in this project:

bakkenl@bakkenl-z01 ~/development/rabbitmq/rabbitmq-packaging (master =)
$ git grep 'var\/run'
RPMS/Fedora/rabbitmq-server.init:27:PID_FILE=/var/run/rabbitmq/pid
RPMS/Fedora/rabbitmq-server.tmpfiles:1:D /var/run/rabbitmq 0755 rabbitmq rabbitmq -
debs/Debian/debian/rabbitmq-server.init:25:PID_FILE=/var/run/rabbitmq/pid

References to /var/run in https://github.com/rabbitmq/rabbitmq-server/

bakkenl@bakkenl-z01 ~/development/rabbitmq/rabbitmq-server (main =)
$ git grep 'var\/run'
deps/rabbit/docs/rabbitmqctl.8:268:.Dl rabbitmqctl wait /var/run/rabbitmq/pid
deps/rabbitmq_peer_discovery_consul/examples/compose_consul_haproxy/docker-compose.yml:26:      - /var/run/docker.sock:/var/run/docker.sock
deps/rabbitmq_peer_discovery_k8s/include/rabbit_peer_discovery_k8s.hrl:36:                                                   default_value = "/var/run/secrets/kubernetes.io/serviceaccount/token"
deps/rabbitmq_peer_discovery_k8s/include/rabbit_peer_discovery_k8s.hrl:41:                                                   default_value = "/var/run/secrets/kubernetes.io/serviceaccount/ca.crt"
deps/rabbitmq_peer_discovery_k8s/include/rabbit_peer_discovery_k8s.hrl:46:                                                   default_value = "/var/run/secrets/kubernetes.io/serviceaccount/namespace"
deps/rabbitmq_prometheus/docker/docker-compose-metrics.yml:70:      - /var/run:/var/run:rw
release-notes/README-2.6.1.txt:10:  /var/run on a temporary file systems, e.g. Ubuntu.
scripts/rabbitmq-server.ocf:39:OCF_RESKEY_pid_file_default="/var/run/rabbitmq/pid"