Specify max SDF version in URDF->SDF conversion

## Desired behavior
`ign sdf -p` should accept a version parameter that would tell it the maximum SDF feature level it can use in the output

## Alternatives considered
Building an old version of sdformat from source... But that's not the solution.

## Additional context
I have a Xacro/URDF model which I would like to use both in Ignition Gazebo and Gazebo classic. But `gz sdf` doesn't understand some tags in the URDF, and `ign sdf` generates the file according to SDF 1.7 spec, which Gazebo 9 doesn't understand. So there's no way for me to use the model in both simulators.