Revert "qt5 client: use openapi 3 spec (#3400)" (#3458)

This reverts commit 40f1f92.

Author: MartinDelille

bin/cpp-qt5-petstore.sh

@@ -27,6 +27,6 @@ fi
 
 # if you've executed sbt assembly previously it will use that instead.
 export JAVA_OPTS="${JAVA_OPTS} -Xmx1024M -DloggerPath=conf/log4j.properties"
-ags="generate -t modules/openapi-generator/src/main/resources/cpp-qt5-client -i modules/openapi-generator/src/test/resources/3_0/petstore.yaml -g cpp-qt5-client -o samples/client/petstore/cpp-qt5 $@"
+ags="generate -t modules/openapi-generator/src/main/resources/cpp-qt5-client -i modules/openapi-generator/src/test/resources/2_0/petstore.yaml -g cpp-qt5-client -o samples/client/petstore/cpp-qt5 $@"
 
 java $JAVA_OPTS -jar $executable $ags