GitHub actions: Update to actions/cache to v4

Author: jamescherti

.github/workflows/ci.yml

@@ -42,12 +42,12 @@ jobs:
       - uses: purcell/setup-emacs@master
         with:
           version: ${{ matrix.emacs-version }}
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         id: cache-cask-packages
         with:
           path: .cask
           key: cache-cask-packages-000
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         id: cache-cask-executable
         with:
           path: ~/.cask