Upt: Update GitHub actions to newer versions

Node 16 actions are deprecated. Hopefully this update will remove the
warning.

.github/actions/build-postfix-exporter/action.yaml

@@ -34,7 +34,7 @@ runs:
         DOCKER_ACCESS_TOKEN: '${{ inputs.DOCKER_ACCESS_TOKEN }}'
 
     - name: Cache Docker layers
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: /tmp/.buildx-cache
         key: ${{ runner.os }}-postfix-exporter-${{ github.sha }}

.github/actions/buildx-setup/action.yaml

@@ -5,7 +5,7 @@ runs:
   using: "composite"
   steps:
     - name: Set up QEMU
-      uses: docker/setup-qemu-action@v2
+      uses: docker/setup-qemu-action@v3
 
     - name: Set up Docker Buildx
       id: buildx