fix failing workflows by using v4 of actions/cache

Author: zaid

.github/workflows/elixir.yml

@@ -18,7 +18,7 @@ jobs:
         elixir-version: '1.15.6'
         otp-version: '26.0'
     - name: Restore dependencies cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: deps
         key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}