UserCore - Add lastActivityOn

What does this MR do and why?

In support of #409313 (closed)
Split off from !135524 (merged)

This change exposes the key lastActivityOn to the UserCoreType. This key was originally only accessible from AddOnUserType. This is being added to support the needs of OrganizationUserType.

Current AddOnUser docs with lastActivityOn: https://docs.gitlab.com/ee/api/graphql/reference/index.html#addonuser

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Access GraphQL explorer (ex. http://127.0.0.1:3000/-/graphql-explorer)
2. Query for lastActivityOn

   query {
     user(username: "root") {
       lastActivityOn
     }
   }

3. Ensure it returns a Date

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.