Bump actions/cache@v2 to v4 following deprecation error by GitHub

Error message from GitHub:

> Error: This request has been automatically failed because it uses a deprecated version of `actions/cache: v2`. Please update your workflow to use v3/v4 of actions/cache to avoid interruptions. Learn more: https://github.blog/changelog/2024-12-05-notice-of-upcoming-releases-and-breaking-changes-for-github-actions/#actions-cache-v1-v2-and-actions-toolkit-cache-package-closing-down

Migration instructions (nothing to do):

actions/cache#1510

Author: fedgiac

.github/workflows/CI.yml

@@ -19,7 +19,7 @@ jobs:
           node-version: ${{ matrix.node }}
       - id: yarn-cache
         run: echo "::set-output name=dir::$(yarn cache dir)"
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           path: ${{ steps.yarn-cache.outputs.dir }}
           key: ${{ matrix.os }}-yarn-${{ hashFiles('**/yarn.lock') }}