The system.tablets table stores replica sets as a CQL set type,
which is sorted. This means that if, in a tablet replica set
[n1, n2, n3] n2 is replaced with n4, then on reload we'll see
[n1, n3, n4], changing the relative position of n3 from the third
replica to the second.
The relative position of replicas in a replica set is important
for materialized views, as they use it to pair base replicas with
view replicas. To prepare for materialized views using tablets,
change the persistent data type to list, which preserves order.
The code that generates new replica sets already preserves order:
see locator::replace_replica().
While this changes the system schema, tablets are an experimental
feature so we don't need to worry about upgrades.
Closes#15111
23be6f0336