(#21427) Deprecate YAML and stop sending it from the agent

YAML has been the cause of many problems. This starts us down the path of
getting rid of it by deprecating all of the places where the master accepts it,
changing the agent to no longer request it, and also changing the agent to no
longer send it (reports and query parameters).

This has the effect that a master *without* these changes will not be able to
correctly understand requests from an agent *with* these changes.