Avoid C++03 testing in clang/libc++

Author: igaztanaga

.github/workflows/ci.yml

@@ -396,13 +396,13 @@ jobs:
           # MacOS, clang
           #------------------
           - toolset: clang
-            cxxstd: "03,11,14,17,20,2b"
+            cxxstd: "11,14,17,20,2b"
             os: macos-13
           - toolset: clang
-            cxxstd: "03,11,14,17,20,2b"
+            cxxstd: "11,14,17,20,2b"
             os: macos-14
           - toolset: clang
-            cxxstd: "03,11,14,17,20,2b"
+            cxxstd: "11,14,17,20,2b"
             os: macos-15
 
     timeout-minutes: 180