Switch to Ubuntu 20 with gcc 7 and clang 6

Author: sbmueller

.github/workflows/c.yaml

@@ -18,14 +18,17 @@ jobs:
         build_type: ["Debug", "Release"]
         compiler:
           [
-            { c: "gcc-14", cxx: "g++-14", package: "gcc-14 g++-14" },
-            { c: "clang-18", cxx: "clang++-18", package: "clang-18" },
+            { c: "gcc-7", cxx: "g++-7", package: "gcc-7 g++-7" },
+            { c: "clang-6", cxx: "clang++-6", package: "clang-6" },
           ]
         build_shared_libraries: [true, false]
 
-    name: "Ubuntu 24.04 (Build: ${{ matrix.build_type }}, Compilers: ${{ matrix.compiler.c }}/${{ matrix.compiler.cxx }}, Shared Library: ${{ matrix.build_shared_libraries }})"
+    name: "Ubuntu 20.04 (Build: ${{ matrix.build_type }}, Compilers: ${{ matrix.compiler.c }}/${{ matrix.compiler.cxx }}, Shared Library: ${{ matrix.build_shared_libraries }})"
 
-    runs-on: ubuntu-24.04
+    # We run on a docker image to avoid being affected from GitHub
+    # decommissioning certain runners
+    runs-on: ubuntu-latest
+    container: ubuntu:20.04
 
     steps:
       - name: Setup