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

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

Author: Lastique

.github/workflows/ci.yml

@@ -467,11 +467,11 @@ jobs:
           - toolset: clang
             cxxstd64: "11,14,17,20,2b"
             instruction_set: "-"
-            os: macos-13
+            os: macos-14
           - toolset: clang
-            cxxstd64: "11,14,17,20,2b"
+            cxxstd64: "11,14,17,20,23"
             instruction_set: "-"
-            os: macos-14
+            os: macos-15
 
     timeout-minutes: 30
     runs-on: ${{matrix.os}}