Use explicit versions of Ubuntu images instead of latest (#484)

To test more versions of compilers

Author: SRombauts

.github/workflows/cmake.yml

@@ -27,8 +27,24 @@ jobs:
             extra_path: "C:\\ProgramData\\chocolatey\\lib\\mingw\\tools\\install\\mingw64\\bin",
           }
         - {
-            name: "Ubuntu Latest GCC",
-            os: ubuntu-latest,
+            name: "Ubuntu 24.04 GCC",
+            os: ubuntu-24.04,
+            generator: "Unix Makefiles",
+            build_type: "Debug",
+            cc: "gcc", cxx: "g++",
+            extra_path: "",
+          }
+        - {
+            name: "Ubuntu 22.04 GCC",
+            os: ubuntu-22.04,
+            generator: "Unix Makefiles",
+            build_type: "Debug",
+            cc: "gcc", cxx: "g++",
+            extra_path: "",
+          }
+        - {
+            name: "Ubuntu 20.04 GCC",
+            os: ubuntu-20.04,
             generator: "Unix Makefiles",
             build_type: "Debug",
             cc: "gcc", cxx: "g++",