Remove all stretch-related configuration switches

roles/base/files/mtail.service

roles/base/files/mtail.service.buster

roles/base/files/node-exporter.default

roles/base/files/node-exporter.default.buster

roles/base/tasks/apt.yml

roles/base/tasks/harden.yml

roles/base/tasks/prometheus.yml

roles/base/tasks/syslog.yml

roles/base/templates/ssh/sshd_config.j2

roles/docker/tasks/buster_upgrade.yml

roles/docker/tasks/main.yml

roles/mariadb/tasks/main.yml

roles/mariadb/templates/exporter.service.j2

test/integration-test-docker.yml