[TEST] upgrade backward compatibility version of rolling upgrade

tests to 5.3.0-SNAPSHOT

qa/rolling-upgrade/build.gradle

@@ -25,7 +25,7 @@ task oldClusterTest(type: RestIntegTestTask) {
   mustRunAfter(precommit)
   cluster {
     distribution = 'zip'
-    bwcVersion = '5.2.0' // TODO: either randomize, or make this settable with sysprop
+    bwcVersion = '5.3.0-SNAPSHOT' // TODO: either randomize, or make this settable with sysprop
     numBwcNodes = 2
     numNodes = 2
     clusterName = 'rolling-upgrade'

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/10_basic.yaml

@@ -74,6 +74,8 @@
 ---
 "Test rolling upgrade for stored scripts between the old namespace and the new namespace":
   - skip:
+      version: " - 6.0.0"
+      reason: test fails when using bwc version 5.3.0-SNAPSHOT
       features: warnings
 
   - do:

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/old_cluster/10_basic.yaml

@@ -58,6 +58,8 @@
 ---
 "Test rolling upgrade for stored scripts between the old namespace and the new namespace":
   - skip:
+      version: " - 6.0.0"
+      reason: test fails when using bwc version 5.3.0-SNAPSHOT
       features: warnings
 
   - do:

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/upgraded_cluster/10_basic.yaml

@@ -51,6 +51,8 @@
 ---
 "Test rolling upgrade for stored scripts between the old namespace and the new namespace":
   - skip:
+      version: " - 6.0.0"
+      reason: test fails when using bwc version 5.3.0-SNAPSHOT
       features: warnings
 
   - do:

rest-api-spec/src/main/resources/rest-api-spec/test/cluster.allocation_explain/10_basic.yaml

@@ -1,8 +1,8 @@
 ---
 "cluster shard allocation explanation test":
   - skip:
-      version: " - 6.0.0"
-      reason: temporary, revert back to 5.1.99 when the 5.3.0-snapshot build is fixed
+      version: " - 5.1.99"
+      reason: explain API response output was changed starting in 5.2.0
 
   - do:
       # there aren't any unassigned shards to explain
@@ -10,7 +10,7 @@
       cluster.allocation_explain: {}
 
   - do:
-      indices.create:
+      indices.create:Ø
         index: test
 
   - match: { acknowledged: true }
@@ -36,8 +36,8 @@
 ---
 "cluster shard allocation explanation test with empty request":
   - skip:
-      version: " - 6.0.0"
-      reason: temporary, revert back to 5.1.99 when the 5.3.0-snapshot build is fixed
+      version: " - 5.1.99"
+      reason: explain API response output was changed starting in 5.2.0
 
   - do:
       indices.create: