[ci] skip R-devel clang16 job and use gcc-12 for macOS Python jobs (#6608)

* [ci] skip R-devel clang16 job

* use gcc-14 on macOS builds

* try gcc-12

.ci/setup.sh

@@ -20,7 +20,7 @@ if [[ $OS_NAME == "macos" ]]; then
         # Check https://github.com/actions/runner-images/tree/main/images/macos for available
         # versions of Xcode
         sudo xcode-select -s /Applications/Xcode_14.3.1.app/Contents/Developer || exit 1
-        brew install gcc
+        brew install 'gcc@12'
     fi
     if [[ $TASK == "mpi" ]]; then
         brew install open-mpi

.ci/test.sh

@@ -14,8 +14,8 @@ ARCH=$(uname -m)
 LGB_VER=$(head -n 1 "${BUILD_DIRECTORY}/VERSION.txt")
 
 if [[ $OS_NAME == "macos" ]] && [[ $COMPILER == "gcc" ]]; then
-    export CXX=g++-11
-    export CC=gcc-11
+    export CXX=g++-12
+    export CC=gcc-12
 elif [[ $OS_NAME == "linux" ]] && [[ $COMPILER == "clang" ]]; then
     export CXX=clang++
     export CC=clang

.github/workflows/r_package.yml

@@ -268,7 +268,9 @@ jobs:
         #   * CRAN "additional checks": https://cran.r-project.org/web/checks/check_issue_kinds.html
         #   * images: https://r-hub.github.io/containers/containers.html
         image:
-          - clang16
+          # clang16 should be re-enabled once it's fixed upstream
+          # ref: https://github.com/microsoft/LightGBM/issues/6607
+          #- clang16
           - clang17
           - clang18
           - clang19