Bump workspace to use Rust 1.91.0

[Jefffrey]

Is your feature request related to a problem or challenge?

Rust 1.91.0 was released today: https://blog.rust-lang.org/2025/10/30/Rust-1.91.0/

Describe the solution you'd like

Bump our toolchain:

datafusion/rust-toolchain.toml

Lines 21 to 23 in 41566db

	[toolchain]
	channel = "1.90.0"
	components = ["rustfmt", "clippy"]

Describe alternatives you've considered

No response

Additional context

Previous bump PR: #17677

[dmefs]

take