fix(ci): Update to clang-format@21

Author: ncooke3

CONTRIBUTING.md

@@ -132,7 +132,7 @@ To develop Firebase software, **install**:
    To install [clang-format] and [mint] using [Homebrew]:
 
     ```console
-    brew install clang-format@20
+    brew install clang-format@21
     brew install mint
     ```
 

README.md

@@ -153,7 +153,7 @@ GitHub Actions will verify that any code changes are done in a style-compliant
 way. Install `clang-format` and `mint`:
 
 ```console
-brew install clang-format@20
+brew install clang-format@21
 brew install mint
 ```
 

scripts/setup_check.sh

@@ -35,7 +35,7 @@ fi
 
 # install clang-format
 brew update
-brew install clang-format@20
+brew install clang-format@21
 
 # mint installs tools from Mintfile on demand.
 brew install mint