Add negative validation to time estimate update mutations

What does this MR do and why?

For #390579 (closed)

Fixes the issue and merge request update mutations to validate the time estimate is not negative and return an error if it is.

Screenshots or screen recordings

updateIssue

updateMergeRequest

How to set up and validate locally

1. Run gdk/gitpod server
2. Visit http://127.0.0.1:3000/-/graphql-explorer
3. Run timeEstimate update mutation for issue, try a negative timeEstimate and ensure error message is returned and estimate is unchanged

mutation {
  updateIssue(
    input: {
      projectPath: "twitter/Typeahead.Js",
      iid: "40",
      timeEstimate: "-1h"
    }
  ) {
    issue {
      id
      timeEstimate
    }
    errors 
  }
}

4. Run timeEstimate update mutation for merge request, try a negative timeEstimate and ensure error message is returned and estimate is unchanged

mutation {
  mergeRequestUpdate(input: {
    iid: "5",
    projectPath: "twitter/Typeahead.Js"
    timeEstimate: "-2h"
  }) {
    mergeRequest {
      id
      timeEstimate
    }
  }
}

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