Add a data-qa-selector for merge_button

This commit adds a data-qa-selector to fix Merge Train end to end tests
Issue https://gitlab.com/gitlab-org/gitlab/-/issues/301110

app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue

@@ -459,6 +459,7 @@ export default {
                 :variant="mergeButtonVariant"
                 :disabled="isMergeButtonDisabled"
                 :loading="isMakingRequest"
+                data-qa-selector="merge_button"
                 @click="handleMergeButtonClick(isAutoMergeAvailable)"
                 >{{ mergeButtonText }}</gl-button
               >