Bump actions/cache from 3 to 4 (#37)

Author: dependabot[bot]

.github/workflows/docs.yml

@@ -16,7 +16,7 @@ jobs:
       - uses: julia-actions/setup-julia@v1
         with:
           version: '1.9'
-      - uses: actions/cache@v3
+      - uses: actions/cache@v4
         with:
           path: |
             ~/.julia/artifacts