Test also c++2c mode

Author: igaztanaga

.github/workflows/ci.yml

@@ -374,7 +374,7 @@ jobs:
             cxxstd: "11,14,17,20,2b"
             os: macos-15
           - toolset: clang
-            cxxstd: "11,14,17,20,2b"
+            cxxstd: "11,14,17,20,23,2c"
             os: macos-26
 
     timeout-minutes: 240