.github: Use the two most recent LTS releases

Ubuntu 24.04 is the most recent LTS release. Use it and the previous
22.04 LTS for cross build testing.

Sponsored by:	Arm Ltd

.github/workflows/cross-bootstrap-tools.yml

@@ -21,14 +21,14 @@ jobs:
         os: [ ubuntu-20.04, ubuntu-22.04, macos-latest ]
         include:
           # TODO: both Ubuntu and macOS have bmake packages, we should try them instead of bootstrapping our own copy.
-          - os: ubuntu-20.04
-            compiler: clang-12
-            cross-bindir: /usr/lib/llvm-12/bin
-            pkgs: bmake libarchive-dev clang-12 lld-12
           - os: ubuntu-22.04
             compiler: clang-14
             cross-bindir: /usr/lib/llvm-14/bin
             pkgs: bmake libarchive-dev clang-14 lld-14
+          - os: ubuntu-24.04
+            compiler: clang-18
+            cross-bindir: /usr/lib/llvm-18/bin
+            pkgs: bmake libarchive-dev clang-18 lld-18
           - os: macos-latest
             compiler: clang-18
             cross-bindir: /opt/homebrew/opt/llvm@18/bin