Github Actions has removed ubuntu-18.04

Replace ubuntu-18.04 virtual environment with ubuntu-20.04.
Compilation tests for GCC < 7 cannot be done anymore
since ubuntu-20.04 environment does not provide such an
old compilers.

Author: temeo

.github/workflows/build.yml

@@ -13,43 +13,19 @@ jobs:
       matrix:
         config:
           # GCC/G++
-          - os: ubuntu-18.04
-            CC: gcc
-            version: "4.8"
-            type: Debug
-          - os: ubuntu-18.04
-            CC: gcc
-            version: "4.8"
-            type: Release
-          - os: ubuntu-18.04
-            CC: gcc
-            version: "5"
-            type: Debug
-          - os: ubuntu-18.04
-            CC: gcc
-            version: "5"
-            type: Release
-          - os: ubuntu-18.04
-            CC: gcc
-            version: "6"
-            type: Debug
-          - os: ubuntu-18.04
-            CC: gcc
-            version: "6"
-            type: Release
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             CC: gcc
             version: "7"
             type: Debug
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             CC: gcc
             version: "7"
             type: Release
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             CC: gcc
             version: "8"
             type: Debug
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             CC: gcc
             version: "8"
             type: Release
@@ -86,11 +62,11 @@ jobs:
             version: "12"
             type: Release
           # Clang
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             CC: clang
             version: "6.0"
             type: Debug
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             CC: clang
             version: "6.0"
             type: Release