Save force_remove_source_branch on click

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

@@ -342,6 +342,7 @@ export default {
         commit_message: this.commitMessage,
         auto_merge_strategy: useAutoMerge ? this.preferredAutoMergeStrategy : undefined,
         should_remove_source_branch: this.removeSourceBranch === true,
+        force_remove_source_branch: this.removeSourceBranch === true,
         squash: this.squashBeforeMerge,
       };