MSRV requires provided lockfile, CI should test more versions of dependency crates #940

We provide two Cargo.lock files, Cargo-{minimal,recent}.lock, but they are (have been?) always the same.

Many of our Cargo.toml version specifications are not semver based (x, or x.y) but exact (x.y.z).

For each toolchain version the ./contrib.test.sh script tests using both lockfiles, which just reruns the tests with the same dependency versions. This means that recently and currently having two lockfiles is just delaying test results.

For better compatibility testing it would be more useful if our version constraints relaxed so that the lockfiles diverged. MSRV would only with the minimal one, whereas more recent toolchains were tested against deps that require more recent versions of rust. See also #774

Secondly, we should cargo test also without using any of the provided lockfiles. Different lockfiles will be produced from what is provided.

MSRV & stable fail without a provided lockfile using cargo from the toolchain (base64ct is an indirect dev dep resulting from the bitcoind crate's dependency on the zip crate):

error: failed to download `base64ct v1.8.0`

Caused by:
  unable to get packages from source

Caused by:
  failed to parse manifest at `/home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/base64ct-1.8.0/Cargo.toml`

Caused by:
  failed to parse the `edition` key

Caused by:
  this version of Cargo is older than the `2024` edition, and only supports `2015`, `2018`, and `2021` editions.

Whereas nightly produces a different lockfile:

$ sha256sum Cargo*.lock
91af669665c87ed8b1d62182df8de837b7305ec7193678b763e69c447c542c72  Cargo-minimal.lock
ced176873f39b270ce00e0eeb4826e302a299f3222e22fb1da89bf944970d991  Cargo-nightly.lock
91af669665c87ed8b1d62182df8de837b7305ec7193678b763e69c447c542c72  Cargo-recent.lock

Third, we should also consider testing against new versions of crates that fit our version constraints, in a scheduled job (maybe once a week?). This would alert us of
potential breakage due to imprecise semver specifications.