CI: use newer Ubuntu for old compilers

Ubuntu 20.04 doesn't seem available anymore.

Author: robUx4

.github/workflows/generate-tools.yaml

@@ -14,8 +14,8 @@ jobs:
         target: [
           {"name": "Linux GCC 14",     "os": "ubuntu-latest",  "cmake_env": "CC=gcc-14"},
           {"name": "Linux Clang 18",   "os": "ubuntu-latest",  "cmake_env": "CC=clang-18"},
-          {"name": "Linux GCC 7",      "os": "ubuntu-20.04",   "cmake_env": "CC=gcc-7",   "install": "gcc-7"},
-          {"name": "Linux Clang 7",    "os": "ubuntu-20.04",   "cmake_env": "CC=clang-7", "install": "clang-7"},
+          {"name": "Linux GCC 7",      "os": "ubuntu-21.04",   "cmake_env": "CC=gcc-7",   "install": "gcc-7"},
+          {"name": "Linux Clang 7",    "os": "ubuntu-21.04",   "cmake_env": "CC=clang-7", "install": "clang-7"},
           {"name": "macOS arm64",      "os": "macos-15",       "cmake_env": "CMAKE_OSX_ARCHITECTURES=arm64"},
           {"name": "macOS x86_64",     "os": "macos-15",       "cmake_env": "CMAKE_OSX_ARCHITECTURES=x86_64"},
           {"name": "Windows",          "os": "windows-latest"},