docs/about/license: Update LICENSE URL

docs/about/license: Update LICENSE URL

In early 2021 (see commit 2ad784339e "docs: update README to use
GitLab repo URLs") almost all of the code base was converted to
point to GitLab instead of git.qemu.org. During 2023, git.qemu.org
switched from a git mirror to a http redirect to GitLab (see [1]).

Update the LICENSE URL to match its previous content, displaying
the file raw content similarly to gitweb 'blob_plain' format ([2]).

[1] https://lore.kernel.org/qemu-devel/CABgObfZu3mFc8tM20K-yXdt7F-7eV-uKZN4sKDarSeu7DYoRbA@mail.gmail.com/
[2] https://git-scm.com/docs/gitweb#Documentation/gitweb.txt-blobplain

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230822125716.55295-1-philmd@linaro.org>