ci: bump actions to v4 in reusable_testing.yml

Bump version of the `actions/checkout` and `actions/download-artifact`
actions to v4 for fixing the following GitHub warning:

    Node.js 16 actions are deprecated.
    Please update the following actions to use Node.js 20

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci

.github/workflows/reusable_testing.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - name: 'Clone the vshard module'
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           repository: ${{ github.repository_owner }}/vshard
           # Fetch the entire history for all branches and tags. It is needed for
@@ -25,7 +25,7 @@ jobs:
           submodules: recursive
 
       - name: 'Download the tarantool build artifact'
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: ${{ inputs.artifact_name }}