Make index rollover action atomic

[dnhatn]

Today when executing a rollover request, we create an index with alias
(via template), then update index aliases in two separate cluster tasks.
In the interval between these two actions, the alias will associate to
two indices. This causes indexing requests to that alias to be rejected.

This commit merges these two actions into a single cluster update task.

Closes #26976

[ywelsch]

I think that the reason for separating index creation and rolling over the alias was for the rollover only to happen after the new index is fully initialized (primaries started), to keep indexing nicely flowing. I could be wrong though, maybe @s1monw can clarify.

[s1monw]

I agree this with @ywelsch this is not the right solution. I think the issue here is that the alisa is modifed by the user in an inappropriate way. To make this error more self explaining I think the rollover API should fail if the Alias we try to roll over contains more than one alias OR if it contains an index pattern. The alias is a write alias as documented and can only have a single index associated with it.

[dnhatn]

Thanks @ywelsch and @s1monw, I am closing it. I will take another approach as @s1monw suggested.