Bump swarmkit to v2

[corhere]

- What I did
Finished what I started with moby/swarmkit#3061

- How I did it
s|"github.com/docker/swarmkit|"github.com/moby/swarmkit/v2|g all .go files outside of the vendor tree (but with VSCode's search and replace, which automatically reformats all touched files)

There was some dependency hell with transitive dependencies and our good friend etcd, but pinning the version of github.com/google/certificate-transparency-go did the trick. A manual upgrade of github.com/opencontainers/image-spec was also required as otherwise go mod tidy would "upgrade" that module to v1.0.2, which lacked a definition that the currently-vendored version of buildkit depends on.

- How to verify it
CI

- Description for the changelog

• Fixed a rare data race in Swarmkit moby/swarmkit@eccc361

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

LGTM

[thaJeztah]

failure was unrelated (kicked CI again)

@tianon @ndeloof @dperny PTAL