Make IID string instead of ID in mr popover query

app/assets/javascripts/mr_popover/queries/merge_request.graphql

-query mergeRequest($projectPath: ID!, $mergeRequestIID: ID!) {
+query mergeRequest($projectPath: ID!, $mergeRequestIID: String!) {
   project(fullPath: $projectPath) {
     mergeRequest(iid: $mergeRequestIID) {
       createdAt

changelogs/unreleased/63261-the-graphql-query-for-the-mr-popover-failes-on-the-frontend.yml

+---
+title: Make sure we are receiving the proper information on the MR Popover by updating
+  the IID in the graphql query
+merge_request:
+author:
+type: fixed