Fix BWC condition on REST test

Adds a missing skip section to a REST test that was forgotten in #21998

rest-api-spec/src/main/resources/rest-api-spec/test/create/30_internal_version.yaml

@@ -21,6 +21,10 @@
 ---
 "Internal versioning with explicit version":
 
+ - skip:
+      version: " - 5.1.1"
+      reason: validation logic only fixed from 5.1.2 onwards
+
  - do:
       create:
           index:          test