Reinstate emr version 6.7.0 in example dags (apache#37729)

A recent change bumped this version but it was (in theory) not required
for the intended fix. Reducing back to this known working version that
is compatible with host patching code for AL2.

tests/system/providers/amazon/aws/example_emr.py

@@ -71,7 +71,7 @@
 
 JOB_FLOW_OVERRIDES: dict[str, Any] = {
     "Name": "PiCalc",
-    "ReleaseLabel": "emr-7.0.0",
+    "ReleaseLabel": "emr-6.7.0",
     "Applications": [{"Name": "Spark"}],
     "Instances": {
         "InstanceGroups": [