[UII] Show count of eligible agents when doing bulk upgrade

[jen-huang]

Follow up to #180791.

When users select agents to bulk upgrade, it is possible to choose a version that is lower than some of the selected agents aka downgrade. Downgrades are currently not allowed.

Currently the behavior is such that we allow these downgrade actions to be sent and let errors surface afterwards in agent activity. This may be surprising for the user. We can make this experience better by following a similar model to Restart upgrade action, where a count of eligible agents for the selected version is shown.

@juliaElastic Could you tell me more about this? Do you mean that currently Restart upgrade is only available for single agents, and we should allow Restart upgrade as a bulk action too?

Restart upgrade is also available as a bulk action. For single agent, Restart upgrade is only enabled if the agent is in updating state, while for bulk selections, the action is always enabled, and when clicking on it, the modal window will show how many agents out of the selection are in updating state, and only those will be included in the action.
This was added here: #166154

Originally posted by @juliaElastic in #180791 (comment)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)