Update autotools.yml

actions/runner-images#6002

Drop Ubuntu 18.04.

Author: amitdo

.github/workflows/autotools.yml

@@ -13,16 +13,6 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - { name: ubuntu-18.04-clang-7-autotools, os: ubuntu-18.04, cxx: clang++-7 }
-          - { name: ubuntu-18.04-clang-8-autotools, os: ubuntu-18.04, cxx: clang++-8 } #installed
-          - { name: ubuntu-18.04-clang-9-autotools, os: ubuntu-18.04, cxx: clang++-9 } #installed
-          - { name: ubuntu-18.04-clang-10-autotools, os: ubuntu-18.04, cxx: clang++-10 }
-
-          - { name: ubuntu-18.04-gcc-7-autotools, os: ubuntu-18.04, cxx: g++-7 } #installed
-          - { name: ubuntu-18.04-gcc-8-autotools, os: ubuntu-18.04, cxx: g++-8 } #installed
-          - { name: ubuntu-18.04-gcc-9-autotools, os: ubuntu-18.04, cxx: g++-9 } #installed
-          - { name: ubuntu-18.04-gcc-10-autotools, os: ubuntu-18.04, cxx: g++-10 } #installed
-
           - { name: ubuntu-20.04-clang-7-autotools, os: ubuntu-20.04, cxx: clang++-7 }
           - { name: ubuntu-20.04-clang-8-autotools, os: ubuntu-20.04, cxx: clang++-8 } #installed
           - { name: ubuntu-20.04-clang-9-autotools, os: ubuntu-20.04, cxx: clang++-9 } #installed