[Infra] Update to clang-format@20 and apply updated styling. (#14531)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

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@19
+    brew install clang-format@20
     brew install mint
     ```
 

scripts/setup_check.sh

@@ -35,7 +35,7 @@ fi
 
 # install clang-format
 brew update
-brew install clang-format@19
+brew install clang-format@20
 
 # mint installs tools from Mintfile on demand.
 brew install mint

scripts/style.sh

@@ -56,7 +56,7 @@ version="${version/ (*)/}"
 version="${version/.*/}"
 
 case "$version" in
-  19)
+  20)
     ;;
   google3-trunk)
     echo "Please use a publicly released clang-format; a recent LLVM release"
@@ -65,13 +65,12 @@ case "$version" in
     exit 1
     ;;
   *)
-    echo "Please upgrade to clang-format version 19."
+    echo "Please upgrade to clang-format version 20."
     echo "If it's installed via homebrew you can run:"
     echo "brew upgrade clang-format"
     exit 1
     ;;
 esac
-
 # Ensure that tools in `Mintfile` are installed locally to avoid permissions
 # problems that would otherwise arise from the default of installing in
 # /usr/local.