Add GitHub workflow checking minimal dependency versions

[Marcono1234]

Problem solved by the enhancement

By accident Struson could depend on a future introduced by a version of a dependency newer than specified in Cargo.toml; users of Struson could then encounter build issues if Cargo's dependency resolution picks the older version (e.g. due to restrictions from other dependencies).

Enhancement description

Consider using Cargo's direct-minimal-versions or https://crates.io/crates/cargo-minimal-versions (or similar).

Alternatives / workarounds

• Include this as task in Makefile.toml instead of just in a GitHub workflow
  (maybe not worth it because it causes too much overhead for every build)
• Do nothing and hope / assume the problem described above is not common enough

[Marcono1234]

Maybe this is not that important, or needed at all, because Dependabot keeps the versions up to date and makes sure Cargo.toml and Cargo.lock are in sync.
Might mainly be an issue when intentionally staying on older minimum dependency version, but the version in Cargo.lock is updated to a newer version.