Update github.com/opencontainers/runtime-tools to v0.6.0

Also add new dependencies on github.com/xeipuuv/gojson* (brought up by
new runtime-tools) and adapt the containerd/cri code to replace the APIs
that were removed by runtime-tools.

In particular, add new helpers to handle the capabilities, since
runtime-tools now split them into separate sets of functions for each
capability set.

Replace g.Spec() with g.Config since g.Spec() has been deprecated in the
runtime-tools API.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

pkg/server/container_execsync.go

@@ -102,7 +102,7 @@ func (c *criService) execInContainer(ctx context.Context, id string, opts execOp
 	if opts.tty {
 		g := newSpecGenerator(spec)
 		g.AddProcessEnv("TERM", "xterm")
-		spec = g.Spec()
+		spec = g.Config
 	}
 	pspec := spec.Process
 	pspec.Args = opts.cmd