[firebase-release] Removed change log and reset repo after 14.21.0 release

Author: google-oss-bot

CHANGELOG.md

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-- Added a command `firebase firestore:databases:clone` to clone a Firestore database (#9262).
-- Added JSON format support for Cloud Functions secrets with `--format json` flag and auto-detection from file extensions (#1745)
-- `firebase dataconnect:sdk:generate` will run `init dataconnect:sdk` automatically if no SDKs are configured (#9325).
-- Tightened --only filter resolution for functions deployment to prefer codebase names (#9353)
-- Fixed an issue where the Data Connect emulator would attempt to export data when `FIREBASE_DATACONNECT_POSTGRESQL_STRING` is set (#9330)
-- Added `functions.disallowLegacyRuntimeConfig` option to `firebase.json` to optionally skip fetching legacy Runtime Config during function deploys (#9354)
-- Updated to v2.15.1 of the Data Connect emulator, which includes internal improvements (#9366)
-- Updated Data Connect Dart SDK's default import path to `import 'dataconnect_generated/generated.dart';` (#9365)