ci(deps): bump extractions/setup-just from 3 to 4

v4 moves the underlying `extractions/setup-crate` pin from v1.4.0
to v2.0.0, which moves the action runtime to Node 24 — supported
natively by GitHub-hosted `ubuntu-latest`. Both documented inputs
(`just-version` and `github-token`) are byte-identical between v3
and v4.

Tag: https://github.com/extractions/setup-just/tree/v4
Releases: https://github.com/extractions/setup-just/releases

Author: tony

.github/workflows/docs.yml

@@ -54,7 +54,7 @@ jobs:
 
       - name: Install just
         if: env.PUBLISH == 'true'
-        uses: extractions/setup-just@v3
+        uses: extractions/setup-just@v4
 
       - name: Cache sphinx fonts
         if: env.PUBLISH == 'true'