Bump actions/setup-node from 3 to 4

Bumps [actions/setup-node](https://github.com/actions/setup-node) from 3 to 4.
- [Release notes](https://github.com/actions/setup-node/releases)
- [Commits](actions/setup-node@v3...v4)

---
updated-dependencies:
- dependency-name: actions/setup-node
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/backend-tests.yml

@@ -26,7 +26,7 @@ jobs:
         with:
           path: ./plugin
       -
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
           node-version: 20
           cache: 'npm'

.github/workflows/npmpublish.yml

@@ -40,7 +40,7 @@ jobs:
           mkdir -p node_modules
           ln -s ../../etherpad-lite/src node_modules/ep_etherpad-lite
       -
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
           node-version: 20
           cache: 'npm'
@@ -83,7 +83,7 @@ jobs:
         with:
           fetch-depth: 0
       -
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
           node-version: 20
           registry-url: https://registry.npmjs.org/