Change artistic motif from 9 to 8
    
    - This change is necessary to preserve visual consistency between
      website and distribution, at this moment of time where CentOS Stream 9
      distribution hasn't been released yet. Please see
      https://wiki.centos.org/ArtWork/Identity.
    
    - Update theme version from 0.8.9 to 0.8.10. See changes in
      https://gitlab.com/areguera/jekyll-theme-centos/-/commit/e0f308aab55b25c4fd6aee58100577e87bbd13c5