Enable searching for MRs by updated_at

What does this MR do and why?

This updates the Merge Request resolvers to add support for updated_ arguments, enabling searching with cutoffs.

This is already supported in the underlying finders and in the REST API, so this brings GraphQL closer to parity with REST.

Example GraphQL queries

query($project: ID!, $time: Time!) {
  project(fullPath: $project) {
    mergeRequests(updatedAfter: $time) {
      nodes {
        title
        description
        updatedAt
      }
    }
  }
}

example variables:

{
  "project": "flightjs/flight",
  "time": "2022-02-02"
}

MR acceptance checklist

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