Removed clang 3.5 and 3.6 from GitHub Actions CI.

Boost.SmartPtr now requires a fully functional <atomic>, and these compilers
fail to compile it.

.github/workflows/ci.yml

@@ -108,20 +108,6 @@ jobs:
               - g++-13
 
           # Linux, clang
-          - toolset: clang
-            compiler: clang++-3.5
-            cxxstd: "11"
-            os: ubuntu-latest
-            container: ubuntu:16.04
-            install:
-              - clang-3.5
-          - toolset: clang
-            compiler: clang++-3.6
-            cxxstd: "11,14"
-            os: ubuntu-latest
-            container: ubuntu:16.04
-            install:
-              - clang-3.6
           - toolset: clang
             compiler: clang++-3.7
             cxxstd: "11,14"