CI(deps): Update dependency sphinx-sitemap to v2.8.0 (OSGeo#6218)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

python/grass/docs/requirements.txt

@@ -1,4 +1,4 @@
 sphinx==8.2.3
-sphinx-sitemap==2.7.2
+sphinx-sitemap==2.8.0
 sphinx-last-updated-by-git==0.3.8
 sphinx-material==0.0.36