Remove testing with Clang 3.5 & 3.5 from GHA

Boost.SmartPtr (used by Boost.Thread) no longer builds on those old compilers.

Author: Flamefire

.github/workflows/ci.yml

@@ -110,8 +110,6 @@ jobs:
               compiler: gcc-8,     cxxstd: '11,2a',          os: ubuntu-latest, container: 'ubuntu:20.04', install: 'g++-8-multilib', address-model: '32,64' }
 
           # Linux, clang
-          - { compiler: clang-3.5, cxxstd: '11',             os: ubuntu-latest, container: 'ubuntu:16.04' }
-          - { compiler: clang-3.6, cxxstd: '11,14',          os: ubuntu-latest, container: 'ubuntu:16.04' }
           - { compiler: clang-3.7, cxxstd: '11,14',          os: ubuntu-latest, container: 'ubuntu:16.04' }
           - { compiler: clang-3.8, cxxstd: '11,14',          os: ubuntu-latest, container: 'ubuntu:16.04' }
           - { compiler: clang-3.9, cxxstd: '11,14',          os: ubuntu-latest, container: 'ubuntu:18.04' }