Update deprecated Xcode in CI (#2114)

This changelist updates a deprecated version of Xcode in our GitHub CI.

Author: jstone-lucasfilm

.github/workflows/main.yml

@@ -68,10 +68,10 @@ jobs:
           cmake_config: -DMATERIALX_BUILD_SHARED_LIBS=ON
           python: 3.9
 
-        - name: MacOS_Xcode_14_Python311
+        - name: MacOS_Xcode_15_Python311
           os: macos-14
           compiler: xcode
-          compiler_version: "14.3"
+          compiler_version: "15.1"
           python: 3.11
           static_analysis: ON
           cmake_config: -DCMAKE_EXPORT_COMPILE_COMMANDS=ON