test: Bump opa to 1.12.3 (#808)

Note: This was missed in stackabletech/docker-images#1378

Author: NickLarsenNZ

tests/test-definition.yaml

@@ -31,7 +31,7 @@ dimensions:
       - 3.9.4
   - name: opa
     values:
-      - 1.4.2
+      - 1.12.3
   - name: hadoop
     values:
       - 3.4.2