Bump MSRV to 1.41

[commonquail]

Cargo 1.41 produces a backwards incompatible lock file format [1], and
Cargo 1.47 automatically migrates old lock files to the new format on
"cargo update" [2]. The former means that any new lock file requires
Cargo 1.38 [3] to build, which didn't work with the current MSRV of 1.36
and broke builds before commit 2981ccd ("Track Cargo.lock", 2020-12-17).
The latter means that already existing lock files will also break; while
cargo-lock [4] can change the format of a lock file, that really isn't a
sustainable workflow.

To avoid both insanity and most breakage we must bump MSRV to at least
1.38. To avoid all breakage we must move to 1.41. That works for
openSUSE Leap 15.2, Ubuntu 16.04, and Debian "buster" (stable).

[1] https://github.com/rust-lang/cargo/blob/rust-1.47.0/CHANGELOG.md#cargo-141-2020-01-30
[2] https://github.com/rust-lang/cargo/blob/rust-1.47.0/CHANGELOG.md#cargo-147-2020-10-08
[3] https://github.com/rust-lang/cargo/blob/rust-1.47.0/CHANGELOG.md#cargo-138-2019-09-26
[4] https://github.com/rustsec/cargo-lock

[commonquail]

Ah, it looks like perhaps 1.40 is the version that produces the new format and 1.38 is the first read support for the transition period.