[CI] Remove deprecated Ubuntu 18.04 builds.

Author: mjpt777

.github/workflows/ci.yml

@@ -141,11 +141,11 @@ jobs:
 
  cpp-clang-build:
  name: C++ Clang ${{ matrix.version }}
- runs-on: ubuntu-18.04
+ runs-on: ubuntu-20.04
  strategy:
  fail-fast: false
  matrix:
- version: [ '6.0', '7', '8', '9', '10', '11', '12', '13', '14' ]
+ version: [ '8', '9', '10', '11', '12', '13', '14' ]
  env:
  CC: clang-${{ matrix.version }}
  CXX: clang++-${{ matrix.version }}