[Upgrade Assistant] Prevent users from reindexing follower indices.

[SoniaSanzV]

Context

In Upgrade Assistant, we are allowing follower indices to be reindexed and it always fails [read more about the bug].

From the issue investigation in ES side:

I think that this behavior has always existed since the CCR feature was implemented. At least I reproduces the same exact errors with CCR follower indices created on 6.8 and upgraded to 7.17 and then 8.18.
Because they are supposed to replicate an index from another cluster, some operations are forbidden and cannot be executed against CCR follower indices. This is the case of direct indexing operations (like Reindexing does) and alias changes.
In that case, the bug lies in the Upgrade Assistant (UA) that should probably not offer the option to reindex follower indices. Or if it does, it should be "Unfollow & Reindex" to allow converting the index to a regular index and then reindex the docs. Converting would mean to pause the replication, close the index, unfollow and reopen the index to allow reindexing. The replication could not be reenabled afterwards, so I'm not sure of the benefits compared to the "Mark as Read-only" option.
Note that "Mark as read-only" option does what it means, but replication is not stopped either but only silently fails.

Solution

Since the option "Unfollow & Reindex" requires more work from Es and Kibana side, as a short term solution we can allow follower indices only to be marked as read-only and display some kind of warning that they should stop following the leader index.

Only for illustrative purposes of what the solution might look like. They are not mocks:

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)