Consider transitioning `gltf-derive` to use `syn@2` instead of `syn@1`

[LikeLakers2]

Consider transitioning `gltf-derive` to use `syn@2` instead of `syn@1`

[Elabajaba]

syn 2 is as easy as just changing it from 1 to 2 in cargo.toml, and everything still seems to work in my limited testing.

The current stated msrv is wrong because image 0.24.0 has an msrv of 1.56, and image 0.24.6 has an msrv of 1.61.

The hard part is that if we're raising the msrv that high, we might as well switch to the 2021 edition, but that breaks compilation. ("trait objects must include the dyn keyword" error on the Validate macro from gltf-derive everywhere it's used in gltf-json, see #355)

edit: Regarding the msrv, image seems to bump it relatively often in patch releases for performance wins so I'm not sure if gltf should even bother with an msrv since it isn't very actively maintained, and it'd be getting version bumps relatively often.