Bump MSRV of futures-util to 1.41

[taiki-e]

This bumps MSRV of futures-util and its dependents (futures, futures-executor, futures-test).

This is because the latest memchr (dependency of futures-util) requires Rust 1.41.

https://github.com/rust-lang/futures-rs/runs/2480445418?check_suite_focus=true

error[E0658]: non exhaustive is an experimental feature
   --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/memchr-2.4.0/src/memmem/prefilter/mod.rs:152:1
    |
152 | #[non_exhaustive]
    | ^^^^^^^^^^^^^^^^^
    |

Also, the motivation behind Rust 1.41 being set as memchr's new MSRV seems reasonable to me:

BurntSushi/memchr@952717e

This also bumps the MSRV to Rust 1.41, which is what is currently in
Debian stable.

Users can continue to use the old compiler by downgrading memchr. However, I don't think futures-util itself needs to do that as I think 1.41 is a reasonable MSRV (it was released over a year ago).