Remove tests on macOS 11

Author: flashcode

.github/workflows/ci.yml

@@ -72,7 +72,6 @@ jobs:
       matrix:
         os:
           - macos-12
-          - macos-11
         config:
           - { name: "gcc", cc: "gcc", cxx: "g++", cmake_options: "" }
           - { name: "clang", cc: "clang", cxx: "clang++", cmake_options: "" }