Cannot run Kibana upgrade assistant (7.x=>8.0) on Security Index

[tvernum]

The Kibana upgrade assistant reindexes to a single _doc type in preparation for the full removal of types.
The security index cannot handle this because it explicitly indexes into the doc type.

We hope and expect that there won't be a standard "security index" in 8.0, rather it will be migrated to a special "system index" with a different internal API that will have different feature/compatibility trade offs, and we will need to provide a special migration process to move the 7.x security index to a 8.0 style system-index.

However, if for some reason that doesn't happen, then we will need to move the security code to use the typeless APIs during the 7.x series so that it is compatible with 8.0, and the Kibana upgrade assistant.

Related: #38740

[elasticmachine]

Pinging @elastic/es-security

[droberts195]

I think this is the same problem as #38796 (comment), which is hopefully fixed by #39256, i.e. moving to typeless APIs in 7.0. However, I have some concerns that there are edge cases where there will be problems: #39256 (comment)

We're aiming to test these edge cases at the beginning of next week so you might want to subscribe to that issue and PR in case we find anything.