Merge pull request #255 from Wenzel/ci/doc/update_cache_action

ci: update actions/cache to v4

Author: Wenzel

.github/workflows/doc.yml

@@ -11,17 +11,17 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v4
       - name: build docs
         run: cargo doc
 
   book:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v4
 
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry