Error to decode a key that was persisted after using go1.15 to compile Notary

[reasonerjt]

I have been using go1.14 to compile notary-signer. When I use go1.15 to compile it, it cannot decode the snapshot key that was persisted in the DB earlier.
More details see:
goharbor/harbor#14932

Seems it was caused by misuse in dvsekhvalnov/jose2go:
golang/go#41089
dvsekhvalnov/jose2go#26

and it has been fixed in the upstream repo
dvsekhvalnov/jose2go@85193dd

I hope notary can bump up to use the new version of dvsekhvalnov/jose2go so the upgrade path won't break when we use go1.15+ to compile notary code.