build(deps-dev): bump @firebase/app from 0.6.21 to 0.6.26 (#1329)

Bumps [@firebase/app](https://github.com/firebase/firebase-js-sdk/tree/HEAD/packages/app) from 0.6.21 to 0.6.26.
- [Release notes](https://github.com/firebase/firebase-js-sdk/releases)
- [Changelog](https://github.com/firebase/firebase-js-sdk/blob/@firebase/app@0.6.26/packages/app/CHANGELOG.md)
- [Commits](https://github.com/firebase/firebase-js-sdk/commits/@firebase/app@0.6.26/packages/app)

---
updated-dependencies:
- dependency-name: "@firebase/app"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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