[TEST] 5.x snapshot build is working again, so update the backwards

compatibility tests for the allocation explain API to include 5.2.0

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

@@ -1,8 +1,7 @@
 ---
 "cluster shard allocation explanation test":
   - skip:
-      # Set to 6.0.0 for now because the 5.2 snapshot builds are failing, causing these tests to run against an old 5.2 build
-      version: " - 6.0.0"
+      version: " - 5.1.99"
       reason: allocation explain api format is different in versions < 5.2.0
 
   - do:
@@ -40,8 +39,7 @@
 ---
 "cluster shard allocation explanation test with empty request":
   - skip:
-      # Set to 6.0.0 for now because the 5.2 snapshot builds are failing, causing these tests to run against an old 5.2 build
-      version: " - 6.0.0"
+      version: " - 5.1.99"
       reason: allocation explain api format is different in versions < 5.2.0
 
   - do: