Remove the concept of "net-new" system indices

[gwbrown]

At one point, we intended to more fully lock down system indices in 8.0 than we ended up doing. However, some new system indices wanted to opt in to the post-8.0 behavior early, with the argument that since these indices were new, they were not subject to BWC guarantees, so "net-new" system indices were introduced in #74186. System data streams are considered "net-new" by default.

Now that we've shipped 8.0.0 having partially locked down system indices, we should remove the "net-new" concept, although I don't know exactly what kind of transition path we need here.

Note that some peculiarities of .geoip_databases caused some notable problems until we introduced this behavior, see #72572 for details. These issues could be addressed in other ways, but if we decide to alter this behavior we should be aware of them and reach out to the Kibana team, as their tests triggered the issue at the time causing some disruption.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)