Update actions/checkout action to v4 (#4572)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Francisco Giordano <fg@frang.io>

.github/workflows/formal-verification.yml

@@ -20,7 +20,7 @@ jobs:
   apply-diff:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Apply patches
         run: make -C certora apply
 
@@ -28,7 +28,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.event_name != 'pull_request' || contains(github.event.pull_request.labels.*.name, 'formal-verification')
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
       - name: Set up environment