[Remote Migration] Allow replica allocation on excluded nodes during mixed mode

[gbbafna]

Describe the bug

OpenSearch Users will cluster's migration from doc rep to remote using exclude allocation mechanism . During this when older nodes hosting replica shards goes down, we won't allow new replicas on remote nodes. This is because primary shards always move first . This will cause cluster to remain in yellow state.

Related component

Storage:Durability

Solution

To fix this, we will need to ignore allocation filters for existing replica shards during migration .

Expected behavior

Cluster should not go yellow during docrep to remote store migration .