Expose CI variable limits through GraphQL

What does this MR do and why?

This MR exposes the new limits on project and group CI variables through ProjectVariableConnectionType and GroupVariableConnectionType.

This is the final MR for #362228 (closed)

Screenshots or screen recordings

GraphQL query:

Result:

How to set up and validate locally

1. Make the following query in /-/graphql-explorer

   query {
     project(fullPath: "root/projecty") {
       ciVariables {
         limit
       }
     }
     group(fullPath: "gitlab-org") {
       ciVariables {
         limit
       }
     }
   }

2. See that the result for both project and group limit is 200. This is the default limit for all groups and projects

MR acceptance checklist

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