Remove dependency on once_cell

[jimblandy]

Much of the once_cell crate's API has now been added to std, so ideally we could remove this dependency. This is blocked on std::sync::OnceLock::get_or_try_init being stabilized:

• Tracking Issue for once_cell_try rust-lang/rust#109737

As the once_cell author commented:

@jimblandy FWIW, if you are not using portable atomic, then probably you don't actually need once_cell crate. Most of the API is now available in std (but not in core), so I am thinking that the primary role of once_cell for the ecosystem right now is exactly to serve as a poly-fill for no-std use-cases.

At the moment, wgpu_core::pool::ResourcePool uses once_cell::sync::OnceCell::get_or_try_init, whose std analog, std::sync::OnceLock::get_or_try_init, is not yet stable.

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PRs:

• refactor: s/once_cell::sync::OnceCell/std::sync::OnceLock (as much as possible) #6400
• Raise MSRV to Rust 1.80 #6105
• trunk...erichdongubler-mozilla:wgpu:erichdongubler/push-wzvnqytlqowy

[ErichDonGubler]

Got a PR roadmap laid out for doing this. Edited the OP with checkboxes so the steps are clear.