oldId should be legacyId

[ThomasKranitsas]

I see in the response of api.topcoder-dev.com/v5/groups/ there is an oldId property. This should be named legacyId like we do in all other services there were upgraded to V5

cc @rootelement

[rootelement]

This will have to be done in a deprecation fashion, where legacyId is added and works either/or with oldId, until we can confirm major uses of oldId are removed, then we can shut it down.