[actions] rename ServiceNow isLegacy property to usesTableApi (or similar)

[pmuellr]

Mike Cote and I discussed some of the aspects of the isLegacy config property that got added to the ServiceNow connector in PR 105440. One of them is that isLegacy doesn't "scale well" over time, if we end up having to make some other change like this later, and have to come up with a isLegacy2 field as well. It's also not very descriptive of what it actually means.

We think rather than use a generic term like "legacy", we should probably be in the mode of describing the scenario more specifically instead. For instance, we could rename this field usesTableApi (the old API), with a default of false and the migration changing the old connectors to set this value to true. No changes to the value, just a change to the name. This makes it both "scale better" for other such properties we might need later, as well as being more descriptive in the code, as well as anyone looking at the raw connector data.

There don't seem to be a lot of references to the property in the actions and triggers_actions_ui plugins, so seems like something that would be straight-forward to fix.

This feels kinda low priority to me, and also that if we don't do it before 7.16, we probably don't want to do it at all, because it would then require ANOTHER migration, which seems like overkill. Perhaps it's just a "lesson learned" about a pattern we should avoid in the future ...

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[mikecote]

cc @gmmorris as this should be triaged now as the issue is only valuable if done in 7.16.

[gmmorris]

@mikecote - no need to block on me, feel free to triage this kind of thing yourself and simply let me know when you've done so.
Adding to ToDo 👍