Replace CI RUST_MIN_VER_PKGS with --workspace --ignore-private.

[xStrom]

The CI RUST_MIN_VER_PKGS configuration variable was meant to track packages that are intended for publishing in order to limit MSRV checks to only those packages. This same goal can be achieved universally without the need for a configuration variable via the --ignore-private cargo-hack option.

The immediate effect on this repo is that xilem_core is no longer part of the MSRV check because it has publish = false. That is fine, because the MSRV check will run just before publishing, triggered by the PR that will remove publish = false, ensuring that the MSRV is accurate before publishing.

[DJMcNab]

Both xilem_core and tree-arena will be checked when their corresponding crates are checked anyway, just not exhaustively with all features.