specify common rust version correctly

[MarcoIeni]

FYI, I think this is the Rust standard way to define a common Rust version
across a workspace.
Feel free to close this PR if it doesn't work well with your current workflow :)

[frol]

@miraclx I need to pick your brain to remind me if there is any problem with this approach of setting the version for the packages 🙏

I am happy to merge it if it works with the CI scripts we have!

[miraclx]

Hard to say what effect this will have considering our dependency on cargo-workspaces, I'll extract a small example and run a test. But this already looks like a solution that significantly simplifies the intent.

[frol]

We don't seem to need cargo-workspaces in the first place. I will try using release-plz for release management in borsh-rs.