remove ubuntu-16.04 matrix entries in CI

.github/workflows/ci.yml

@@ -17,10 +17,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - toolset: gcc-4.8
-            cxxstd: "03,11"
-            os: ubuntu-16.04
-            install: g++-4.8
           - toolset: gcc-7
             cxxstd: "03,11,14,17"
             os: ubuntu-18.04
@@ -33,11 +29,6 @@ jobs:
             cxxflags: "cxxflags=--coverage -fsanitize=address,leak,undefined -fno-sanitize-recover=undefined"
             linkflags: "linkflags=--coverage -lasan -lubsan"
             gcov_tool: "gcov-10"
-          - toolset: clang
-            compiler: clang++-3.7
-            cxxstd: "03,11,14"
-            os: ubuntu-16.04
-            install: clang-3.7
           - toolset: clang
             compiler: clang++-9
             cxxstd: "03,11,14,17,2a"