gitlab: Update msys2-64bit runner tags

gitlab: Update msys2-64bit runner tags

Gitlab has deprecated and removed support for windows-1809
and shared-windows.  Update to saas-windows-medium-amd64 per

https://about.gitlab.com/blog/2024/01/22/windows-2022-support-for-gitlab-saas-runners/

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20240507175356.281618-1-richard.henderson@linaro.org>