Fix upgraded_scroll test (#48525)

I think the problem is that the master is trying to relocate the "upgraded_scroll" shard back to
the node on which it was previously allocated, but to which it can't be allocated now due to the
shard lock being held because of an in-progress scroll. As the master keeps on retrying and
retrying (and indefinitely tries so because max_retries does not apply to relocations, it blocks
any other lower-prioritized task from completing, which leads to the rolling upgrade tests failing
(see #48395). 

Closes #48395

Author: ywelsch

x-pack/qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/10_basic.yml

@@ -6,10 +6,10 @@
         wait_for_active_shards: all
         body:
           settings:
-# we use 1 replica to make sure we don't have shards relocating. Relocating a shard with
-# a scroll on it prevents shards from moving back into a where a scroll is running (it holds the shard lock)
-# see https://github.com/elastic/elasticsearch/issues/31827
-            number_of_replicas: 1
+# we make sure we don't have shards relocating. Relocating a shard with a scroll on it prevents shards from moving back into a where
+# a scroll is running (it holds the shard lock), see https://github.com/elastic/elasticsearch/issues/31827
+            index.routing.rebalance.enable: none
+            index.number_of_replicas: 0
             index.routing.allocation.include.upgraded: true
 
   - do: