Update integrations images build script.

Remove pre-2.5 image builds commands since the minimum Ruby
version we now support is 2.5.

Add 3.4 as a supported Ruby version to the help text
(it is already handled by the script).

integration/script/build-images

@@ -14,7 +14,7 @@ while getopts ":hv:" opt; do
       echo "Usage: ./script/build-images [-v RUBY_VERSION]"
       echo
       echo "If no Ruby version is specified, images are built for each of the"
-      echo "supported versions (currently 2.1 through 3.3)."
+      echo "supported versions (currently 2.5 through 3.4)."
       exit 0
       ;;
     v)
@@ -39,10 +39,6 @@ docker build -t datadog/dd-apm-demo:agent -f $INTEGRATION_DIR/images/agent/Docke
 if test -n "$APP_RUBY_VERSION"; then
   docker build -t datadog/dd-apm-demo:rb-$APP_RUBY_VERSION -f $INTEGRATION_DIR/images/ruby/$APP_RUBY_VERSION/Dockerfile $INTEGRATION_DIR/images
 else
-  docker build -t datadog/dd-apm-demo:rb-2.1 -f $INTEGRATION_DIR/images/ruby/2.1/Dockerfile $INTEGRATION_DIR/images
-  docker build -t datadog/dd-apm-demo:rb-2.2 -f $INTEGRATION_DIR/images/ruby/2.2/Dockerfile $INTEGRATION_DIR/images
-  docker build -t datadog/dd-apm-demo:rb-2.3 -f $INTEGRATION_DIR/images/ruby/2.3/Dockerfile $INTEGRATION_DIR/images
-  docker build -t datadog/dd-apm-demo:rb-2.4 -f $INTEGRATION_DIR/images/ruby/2.4/Dockerfile $INTEGRATION_DIR/images
   docker build -t datadog/dd-apm-demo:rb-2.5 -f $INTEGRATION_DIR/images/ruby/2.5/Dockerfile $INTEGRATION_DIR/images
   docker build -t datadog/dd-apm-demo:rb-2.6 -f $INTEGRATION_DIR/images/ruby/2.6/Dockerfile $INTEGRATION_DIR/images
   docker build -t datadog/dd-apm-demo:rb-2.7 -f $INTEGRATION_DIR/images/ruby/2.7/Dockerfile $INTEGRATION_DIR/images