Target specs v3

[Firestar99]

• ci: test interesting rust-gpu revs #129 tests with target specs v2
• upgrades spirv-builder to after Target specs v3 rust-gpu#484
  • to test that PR on various rust-gpu versions in ci
• removes all target spec code in cargo-gpu, all of it is now handled in spirv-builder
• rust-gpu versions after PR #484 will still work with older cargo-gpu versions, until we update target specs in rust-gpu again (not planned anytime soon)
• see https://github.com/Rust-GPU/cargo-gpu/actions/runs/20230926648 succeeding with "the hack" no longer necessary

[tombh]

Looks good to me. I can approve but maybe others will have better context?