Connections
Prevents problems like the one described in #7526.

Description
This PR adds a CI job which confirms building with cargo update -Zdirect-minimal-versions on some platforms. This unstable feature updates all dependencies from the workspace packages to non-workspace packages to the minimum version allowed by the workspace’s dependency specifications, thus ensuring that the project will not build if it actually depends on items newer than its dependency specifications.

It also updates dependency versions to be compatible with -Zdirect-minimal-versions. Specifically, cargo update -Zdirect-minimal-versions will fail if any transitive dependency has a higher dependency requirement than the workspace. This should largely make no difference since those versions would have been required anyway; however, it may increase the minimum dependency requirement seen by users of a single package from crates.io (e.g. users of naga without wgpu). That could be a reason to reject this change.

Also, this change will increase the Cargo.toml maintenance burden, because increasing some dependency versions may require also increasing others even when that change is a noop for any normal build.

Testing
I confirmed that tests pass with the dependencies actually updated, except for naga back::msl::writer::test_stack_size. However, the added CI job does not run tests, only cargo check, to minimize the additional CI cost. Thus, CI will catch use of newer items, but not bugs in old dependencies.

Squash or Rebase?
Rebase

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.