[Uptime UI] Add notification for synthetic agent upgrades to UI

[paulb-elastic]

Similarly to the requirement for User Experience (RUM), we need to inform users to upgrade if the Heartbeat/Synthetic agent is older than the Kibana stack version.

@elastic/observability-design, are you ok for us to follow the same design approach as in User Experience, or would you rather a separate design issue raised?

Implementation consideration, there could be multiple synthetic monitors running from different agents, some old and some up to date. What should the check do (being mindful of what we can get efficiently in such a query), for example:

• look for any results from ‘today’ (irrespective of the current date/time picker) and show the message if there are old agent results (this is useful if looking at historic results, which could easily be from old agents, which would give a potentially confusing message to the user)
• look for any results in the date/time range selected (could be misleading, as per the comment above)

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)