validate: Don't pretend to support other spec versions
validate: Don't pretend to support other spec versions

I'm not sure how folks plan on implementing multi-version support in
Go, but we certainly don't have it now.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/runtime-tools,liangchenye/ocitools,hmeng-19/ocitools,opencontainers/runtime-tools,hmeng-19/ocitools,opencontainers/ocitools,wking/ocitools-v2,liangchenye/ocitools,opencontainers/ocitools,wking/ocitools-v2