ci: add check that ensures we bump a dependency's version

[thomaseizinger]

In #4107, we forgot to bump the patch version of libp2p-identity. This is something that happened several times already. Just after we release a crate, we need to bump the patch version on the next PR that makes it into master.

We should add a CI check that prevents this from happening. There are several ways of solving this but I am thinking:

• Add an additional step to our per crate job that is conditional on the CHANGELOG.md of the crate. The assumption here is that if we touch the changelog, we also need to bump the version if it hasn't happened yet.
• As a next step, fetch the current version from crates.io. If it is different from our version in the manifest, fail CI.

Alternative ideas welcome.

[thomaseizinger]

• Add an additional step to our per crate job that is conditional on the CHANGELOG.md of the crate. The assumption here is that if we touch the changelog, we also need to bump the version if it hasn't happened yet.

Actually, we should probably just check if anything within that crate's directory changed.

[thomaseizinger]

Duplicate of #4367.