build: add missing breakingChangeLabel in ng-dev config

Currently, we are unable to merge breaking changes, because the label doesn't match the default value set in https://github.com/angular/angular/blob/1684b70b8830684deae4bae20c1e12aeddc4e1df/dev-infra/pr/merge/pull-request.ts#L23

(cherry picked from commit b2411ef)

Author: alan-agius4

.ng-dev/merge.ts

@@ -14,6 +14,7 @@ export const merge: DevInfraMergeConfig['merge'] = async (api) => {
       labels: [{ pattern: 'squash commits', method: 'squash' }],
     },
     claSignedLabel: 'cla: yes',
+    breakingChangeLabel: 'flag: breaking change',
     mergeReadyLabel: /^action: merge(-assistance)?/,
     caretakerNoteLabel: /(action: merge-assistance)/,
     commitMessageFixupLabel: 'commit message fixup',