chore: Use Rust 1.80; drop once_cell

[joshuamegnauth54]

libcosmic already requires Rust 1.80.

[bbb651]

Was just about to do that after dropping it in my app xD

I don't think adding MSRV is necessary, since it's a binary crate and not a library, and it'll already fail on libcosmic on <1.80 regardless. And if someone didn't add it they might not know or remember to update it, especially with libcosmic updates that bump it, and imo an incorrect MSRV is worst than no MSRV at all

[joshuamegnauth54]

Good point. I'll remove the MSRV since it doesn't really matter here.