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

@@ -309,12 +309,12 @@ jobs:
               - libc++-18-dev
               - libc++abi-18-dev
 
-          - toolset: clang
-            cxxstd: "11,14,17,20,2b"
-            os: macos-13
           - toolset: clang
             cxxstd: "11,14,17,20,2b"
             os: macos-14
+          - toolset: clang
+            cxxstd: "11,14,17,20,23"
+            os: macos-15
 
     timeout-minutes: 15
     runs-on: ${{matrix.os}}