Deprecating glTF 1.0 support

[Kupoman]

With support and interest in glTF 1.0 dropping in the community, we have decided to deprecate support for it in version 1.2.0 of blendergltf. We will continue to fix small glTF 1.0 export bugs through 1.2.x releases, but the next major or minor release will likely drop support for glTF 1.0 export entirely. This issue is being opened to allow for discussion on this topic. If anyone is currently using this exporter for glTF 1.0 output, now is your chance to speak up.