xtask: check crates.io for reserved versions before publishing

[JurajSadel]

Release tooling could select permanently reserved crates.io versions because it only checked Cargo.toml. plan and execute-plan now check crates.io first and publish-plan re-checks before uploading.

closes #5385

[bjoernQ]

I guess the check should only be done for plan? It's kind of unlikely we yank / release between the steps

At least checking right before cargo publish seems unnecessary - cargo publish would tell us the very same thing and even worse ... at that point we already released a few crates with the potentially wrong dependency version?

[bjoernQ]

Or maybe the check should be in / before execute-plan? Not sure if we really want that "auto-choose a free version" - maybe just fail and let the human decide how to change the release plan?

But I'm not too familiar with the release process - maybe I'm missing something here