MSRV check broken

[rursprung]

the CI tests on rust 1.63 to ensure MSRV but it doesn't use a Cargo.lock. since not all crates have strict MSRV policies it can happen that a newer version of a dependency is being pulled in which breaks the CI for MSRV.

this is exactly what just happened for #122: error: package tokio-macros v2.4.0 cannot be built because it requires rustc 1.70 or newer, while the currently active rustc version is 1.63.0 (see the full build output)

there are three ways to deal with this:

• slacken the MSRV policy (defmt style)
• check in Cargo.lock (see the cargo FAQ & blog post)
• check in a copy of Cargo.lock specific for the MSRV check (e.g. as Cargo.lock.msrv) and copy it to Cargo.lock as part of the CI script (downside: it has to be manually maintained if dependencies are added/updated by copying over the correct Cargo.lock to Cargo.lock.msrv to commit it)

[dbrgn]

The strict MSRV policy is still from a time where we had very minimal dependencies. By now we include both eh0 and eh1, and also the whole async jungle, which makes our current MSRV policy unrealistic.

Let's move to a less strict MSRV policy.