It's very easy to use new rust features without realizing it since
the CI and local development workflows may use the latest rust version.
We try to keep some backwards compatibility with rust versions to make
packaging easier for some OS-level package-managers like Void Linux's.
See #1881.
This change runs the "Check" step for the pinned version of rust in
the rust-toolchain.toml file as well as the MSRV version in a matrix.
In order to bump the MSRV, we need to edit
.github/workflows/msrv-rust-toolchain.toml
This commit sets the MSRV as 1.60.0 but a later child commit will
reduce the MSRV back to 1.57.0.
Closes#2482.
