Upgrade clang-format to v18

[andrewheard]

clang-format@17 is no longer available in Homebrew, upgraded to v18.

#no-changelog

[ncooke3]

Thanks!

2 other files:

git grep clang-format@
CONTRIBUTING.md:    brew install clang-format@17
README.md:brew install clang-format@17

[andrewheard]

2 other files:

git grep clang-format@
CONTRIBUTING.md:    brew install clang-format@17
README.md:brew install clang-format@17

Thanks! Done.

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestoreInternal.framework

  Overall coverage changed from 88.17% (2800722) to 88.10% (67800f9) by -0.07%.

  Filename	Base (2800722)	Merge (67800f9)	Diff
  exception.cc	84.21%	23.68%	-60.53%
  leveldb_key.cc	98.14%	98.82%	+0.69%
  leveldb_remote_document_cache.cc	96.55%	94.83%	-1.72%

Test Logs

• Base (2800722): https://github.com/firebase/firebase-ios-sdk/actions/runs/8161374045
• Merge (67800f9): https://github.com/firebase/firebase-ios-sdk/actions/runs/8180106993

1. https://storage.googleapis.com/firebase-sdk-metric-reports/91kf4NkD2x.html