[docs] Use version variables

[colleenmcginnis]

Use version variables from the central configuration instead of relying on docset-level version-related subs so we don't have to update those values in this repo for every release.

Note: This is currently blocked by elastic/docs-builder#1605.

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

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• docs/reference/auditbeat/auditbeat-installation-configuration.md
• docs/reference/auditbeat/auditbeat-template.md
• docs/reference/auditbeat/command-line-options.md
• docs/reference/auditbeat/http-endpoint.md
• docs/reference/auditbeat/load-kibana-dashboards.md
• docs/reference/auditbeat/logstash-output.md
• docs/reference/auditbeat/running-on-docker.md
• docs/reference/auditbeat/running-on-kubernetes.md
• docs/reference/auditbeat/setup-repositories.md
• docs/reference/filebeat/command-line-options.md

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• docs/reference/filebeat/filebeat-input-httpjson.md
• docs/reference/filebeat/filebeat-installation-configuration.md
• docs/reference/filebeat/filebeat-template.md
• docs/reference/filebeat/filtering-enhancing-data.md
• docs/reference/filebeat/http-endpoint.md
• docs/reference/filebeat/load-kibana-dashboards.md
• docs/reference/filebeat/logstash-output.md
• docs/reference/filebeat/running-on-cloudfoundry.md
• docs/reference/filebeat/running-on-docker.md
• docs/reference/filebeat/running-on-kubernetes.md
• docs/reference/filebeat/setup-repositories.md
• docs/reference/heartbeat/command-line-options.md
• docs/reference/heartbeat/heartbeat-installation-configuration.md
• docs/reference/heartbeat/heartbeat-template.md
• docs/reference/heartbeat/http-endpoint.md
• docs/reference/heartbeat/logstash-output.md
• docs/reference/heartbeat/running-on-docker.md
• docs/reference/heartbeat/running-on-kubernetes.md
• docs/reference/heartbeat/setup-repositories.md
• docs/reference/loggingplugin/log-driver-configuration.md
• docs/reference/loggingplugin/log-driver-installation.md
• docs/reference/loggingplugin/log-driver-usage-examples.md
• docs/reference/metricbeat/command-line-options.md
• docs/reference/metricbeat/http-endpoint.md
• docs/reference/metricbeat/load-kibana-dashboards.md
• docs/reference/metricbeat/logstash-output.md
• docs/reference/metricbeat/metricbeat-installation-configuration.md
• docs/reference/metricbeat/metricbeat-template.md
• docs/reference/metricbeat/running-on-cloudfoundry.md
• docs/reference/metricbeat/running-on-docker.md
• docs/reference/metricbeat/running-on-kubernetes.md
• docs/reference/metricbeat/setup-repositories.md
• docs/reference/packetbeat/command-line-options.md
• docs/reference/packetbeat/configuration-flows.md
• docs/reference/packetbeat/http-endpoint.md
• docs/reference/packetbeat/load-kibana-dashboards.md
• docs/reference/packetbeat/logstash-output.md
• docs/reference/packetbeat/packetbeat-installation-configuration.md
• docs/reference/packetbeat/packetbeat-template.md
• docs/reference/packetbeat/running-on-docker.md
• docs/reference/packetbeat/setup-repositories.md
• docs/reference/winlogbeat/command-line-options.md
• docs/reference/winlogbeat/http-endpoint.md
• docs/reference/winlogbeat/load-kibana-dashboards.md
• docs/reference/winlogbeat/logstash-output.md
• docs/reference/winlogbeat/winlogbeat-template.md