Lock version of Datadog agent

The installation of versions 7.57.x fail due to an error unpacking the
software archive. Locking the version to the 7.56 release avoids this
problem for the time being, but we should figure out the actual cause
soon.

Author: jdno

ansible/playbooks/dev-desktop.yml

@@ -17,6 +17,7 @@
         datadog_api_key: "{{ vars_datadog_api_key }}"
         datadog_site: "datadoghq.com"
 
+        datadog_agent_version: "7.56.2"
         datadog_config:
           tags:
             - "env:{{ vars_environment }}"