Update cargo's changelog location

[ehuss]

Cargo has moved its changelog to the static site in order to deal with GitHub markdown rendering issues. This updates all the links to use the new location.

Rendered

[cuviper]

Looks good, and given that the old links are no longer valid, we should merge this right away.

It's probably also worth updating the older rust-relnote links, but at least those aren't totally gone yet, as long as you dodge the GitHub unicorn.

[ehuss]

Thanks @cuviper! Can you click the approve button?

[cuviper]

(sorry about that -- I hit auto-merge thinking it was a merge queue, not waiting on formal approval)