Bump SDF to 1.7 or later

[PeterBowman]

Frame semantics have changed a bit, so relative transformations between frames can simplify things:

• http://sdformat.org/spec?ver=1.6
• http://sdformat.org/spec?ver=1.7
• http://sdformat.org/tutorials?tut=pose_frame_semantics
• https://github.com/gazebosim/sdformat/tree/sdf13/sdf/1.6
• https://github.com/gazebosim/sdformat/tree/sdf13/sdf/1.7

Gazebo 11 is compatible with SDF format spec 1.7 (ref).

[PeterBowman]

The old repo is obsolete, therefore I have transferred it here. In case we need it, changes applied here can be ported back to the old Gazebo classic models.

By the way, Gazebo Harmonic (Ubuntu 24.04, ROS Jazzy) seems to comply with SDFormat 14, which in turns map to protocol version 1.11 (ref, ref).