ci: drop deprecated macos-13 (intel) platform, add macos-15-intel (#2031)

Signed-off-by: Larry Gritz <[email protected]>

Author: lgritz

.github/workflows/ci.yml

@@ -536,15 +536,14 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - desc: MacOS-13-Intel llvm17 oiio-release
-            runner: macos-13
-            nametag: macos13-p313
+          - desc: MacOS-15-Intel aclang17/C++17/py3.13 llvm17 oiio-release
+            runner: macos-15-intel
+            nametag: macos15-p313
             cc_compiler: clang
             cxx_compiler: clang++
             cxx_std: 17
             openimageio_ver: dev-3.0
             python_ver: "3.13"
-            aclang: 14
             ctest_test_timeout: 120
             setenvs: export HOMEBREW_PREFIX=/usr/local
                             LLVMBREWVER="@17"
@@ -557,7 +556,6 @@ jobs:
             cxx_std: 17
             openimageio_ver: main
             python_ver: "3.13"
-            aclang: 15
             setenvs: export LLVMBREWVER="@19"
           - desc: MacOS-15-ARM aclang16/C++17/py3.13 llvm19 oiio-main
             runner: macos-15