[Fleet] Migrate to ScopedHistory router

[joshdover]

Fleet is currently using it's own hash-based router on the frontend, which is not actually supported by Core's ApplicationService. There are number of routing bugs that come from using the hash-based router, but it also blocks #86171.

An example of how this router should be hooked up can be found here: https://github.com/elastic/kibana/blob/master/docs/development/core/public/kibana-plugin-core-public.appmountparameters.history.md

It appears that Fleet is using two routers? A hash-based one inside of the one hooked up to Core's provided history instance. I'm not sure of the motivation for doing this? Location in Fleet where the hash router is used: https://github.com/elastic/kibana/blob/master/x-pack/plugins/fleet/public/applications/fleet/app.tsx#L207

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[elasticmachine]

Pinging @elastic/fleet (Feature:Fleet)

[jfsiii]

cc @paul-tavares bc I know you have experience here. I'm curious if you have any guess about the level of effort to replace? It's not your responsibility, I just know you've had a few go rounds with our routing.