RUST-2216 bump MSRV to 1.81.0

[kevinAlbs]

Summary

Bump MSRV to 1.81.0.

Motivation

To fix failing compile-on-msrv task:

error: package `bson v2.14.0 (https://github.com/mongodb/bson-rust?branch=main#806d68f5)` cannot be built because it requires rustc 1.81 or newer, while the currently active rustc version is 1.74.0

Tested with this patch build: https://spruce.mongodb.com/version/682b3c30e30473000763fd53

mongodb/bson-rust@806d68f added rust-version = "1.81" to Cargo.toml in bson-rust.

https://releases.rs/docs/1.81.0/ was released 5 September, 2024. I expect bumping to 1.81.0 is OK with the MSRV policy:

will be to a Rust version at least six months old.

[isabelatkinson]

I actually put up a fix for this last week (#1373) - the MSRV resolver doesn't work for the way we specify these dependencies, but we technically don't need to do a bump here

[kevinAlbs]

Ah perfect. Thank you for the fix. Closing.