Werk #14342: agent_prometheus & prometheus_build_info: change to display version of main instance

Previously, the agent returned an error if the build_info PromQL returned multiple entries. This effectively prevented the user from using the agent for a Prometheus cluster setup. This werk removes this limitation and adjusts the Prometheus Build Service accordingly. The customization is only visible for Prometheus instances running below version 2.14 and the details can be found in the updated man page

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