Replaced macos-13 with macos-15 in GitHub Actions.

The macos-13 image is retired, macos-15 is now available.

.github/workflows/ci.yml

@@ -464,14 +464,14 @@ jobs:
             install:
               - g++-13
 
-          - toolset: clang
-            cxxstd64: "11,14,17,20,2b"
-            instruction_set: "-"
-            os: macos-13
           - toolset: clang
             cxxstd64: "11,14,17,20,2b"
             instruction_set: "-"
             os: macos-14
+          - toolset: clang
+            cxxstd64: "11,14,17,20,23"
+            instruction_set: "-"
+            os: macos-15
 
     timeout-minutes: 30
     runs-on: ${{matrix.os}}