Compilers used change as follows:
Linux native GCC 7.5 -> GCC 8.4
Linux cross GCC 8.4 -> GCC 8.4
Windows mingw-w64 7.3 -> mingw-w64 9.3
macOS Clang 8.0.0 -> Clang 8.0.0
The macOS and Win cross builds in the CI are updated to use Focal, and
per the op, running the security tests is disabled in the Windows
build.