Bump actions/upload-pages-artifact patch ver to 1.0.8

This changes the internally used action of upload-artifact from `@main` to
`@v3`. Because v4 has been recently published, which introduces quite a
few performance improvements but may be in some cases incompatible with
prior artifact creation workflow, we stick with the v3 that we know
works.

Note: There was 1.0.9 published with a change that's been reverted in
1.0.10, so we might as well upgrade to 1.0.10; the only change was the
back-and-forth fix (breaking change; introduced in 2.0.0) and docs
change. We might upgrade to 2.0.0 but we'd need to verify if the
breaking change of not running `chmod` affects us:

https://github.com/actions/upload-pages-artifact/pull/69/files

.github/workflows/_jobs_github_pages.yml

@@ -49,7 +49,7 @@ jobs:
         uses: "actions/[email protected]"
 
       - name: "Upload artifacts"
-        uses: "actions/[email protected].7"
+        uses: "actions/[email protected].8"
         with:
           path: "documentation/target/site" # _SLANG_MKDOCS_DOCUMENTATION_SITE_DIR_ (keep in sync)