Rename refers field to subject

[sajayantony]

Fixes correspond to the the image spec changes

• https://github.com/opencontainers/image-spec/pull/950

Signed-off-by: Sajay Antony sajaya@microsoft.com

[sudo-bmitch]

CI should be fixed if this is rebased.

[sajayantony]

Rebased and CI is now passing. Please help review this again @vbatts @sudo-bmitch @dmcgowan @jdolitsky.

[mikebrow]

reads much better, I think.. Thx @sajayantony