text: Title: [actions] rename ServiceNow `isLegacy` property to `usesTableApi` (or similar)
  Body: Mike Cote and I discussed some of the aspects of the `isLegacy` config property that got added to the ServiceNow connector in [PR 105440](https://github.com/elastic/kibana/pull/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 ...