Switch to macos-11 GHA image as macos-10.15 is deprecated.

Author: Lastique

.github/workflows/ci.yml

@@ -258,7 +258,7 @@ jobs:
 
           - toolset: clang
             cxxstd: "03,11,14,17,2a"
-            os: macos-10.15
+            os: macos-11
 
           - name: CMake tests
             cmake_tests: 1