GitHub cross-build CI: Update compiler versions

Use the latest Clang packages available for macos and Ubuntu 24.04 to
ensure we also test newer versions in addition to the oldest supported
version.

Author: arichardson

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

@@ -26,13 +26,13 @@ jobs:
             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
+            compiler: clang-20
+            cross-bindir: /usr/lib/llvm-20/bin
+            pkgs: bmake libarchive-dev clang-20 lld-20
           - os: macos-latest
-            compiler: clang-18
-            cross-toolchain: llvm@18
-            pkgs: bmake libarchive llvm@18
+            compiler: clang-21
+            cross-toolchain: llvm@21
+            pkgs: bmake libarchive llvm@21
           - target_arch: amd64
             target: amd64
           - target_arch: aarch64