build: bump github.com/firebase/firebase-ios-sdk from 11.14.0 to 11.15.0 (#84)

Bumps [github.com/firebase/firebase-ios-sdk](https://github.com/firebase/firebase-ios-sdk) from 11.14.0 to 11.15.0.
- [Release notes](https://github.com/firebase/firebase-ios-sdk/releases)
- [Commits](firebase/firebase-ios-sdk@11.14.0...11.15.0)

---
updated-dependencies:
- dependency-name: github.com/firebase/firebase-ios-sdk
  dependency-version: 11.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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