Replaced macos-11 GHA image with 12, 13 and 14 since 11 is being removed.

Author: Lastique

.github/workflows/ci.yml

@@ -270,8 +270,14 @@ jobs:
               - libc++abi-15-dev
 
           - toolset: clang
-            cxxstd: "11,14,17,2a"
-            os: macos-11
+            cxxstd: "11,14,17,20,2b"
+            os: macos-12
+          - toolset: clang
+            cxxstd: "11,14,17,20,2b"
+            os: macos-13
+          - toolset: clang
+            cxxstd: "11,14,17,20,2b"
+            os: macos-14
 
           - name: CMake tests
             cmake_tests: 1