Remove gcc 4.8 and 4.9 GitHub Actions jobs.

These compilers don't provide std::is_trivially_copyable type trait,
which is now required.

Author: Lastique

.github/workflows/ci.yml

@@ -33,18 +33,6 @@ jobs:
       matrix:
         include:
           # Linux, gcc
-          - toolset: gcc-4.8
-            cxxstd: "11"
-            os: ubuntu-latest
-            container: ubuntu:18.04
-            install:
-              - g++-4.8
-          - toolset: gcc-4.9
-            cxxstd: "11"
-            os: ubuntu-latest
-            container: ubuntu:16.04
-            install:
-              - g++-4.9
           - toolset: gcc-5
             cxxstd: "11,14,1z"
             os: ubuntu-latest