roachtest: increase timeout for ChangeReplicasMixedVersion test

We've increased the timeout for individual steps here recently:
- #149243
- #151199

The second PR there codifies the expectation that a single moveReplicas
step may take upwards of 10 minutes, even though this isn't the common
case scenario. Given we move replicas off all nodes, and we do a
moveReplicas step on each binary upgrade, the previous 1 hour limit was
a tad conservative. We bump this to 3 hours.

Closes #159138
Release note: None

Author: arulajmani

pkg/cmd/roachtest/tests/mixed_version_change_replicas.go

@@ -36,7 +36,7 @@ func registerChangeReplicasMixedVersion(r registry.Registry) {
 		Monitor:          true,
 		Randomized:       true,
 		Run:              runChangeReplicasMixedVersion,
-		Timeout:          60 * time.Minute,
+		Timeout:          3 * time.Hour,
 	})
 }