Bump actions/setup-node from 4 to 6

Bumps [actions/setup-node](https://github.com/actions/setup-node) from 4 to 6.
- [Release notes](https://github.com/actions/setup-node/releases)
- [Commits](actions/setup-node@v4...v6)

---
updated-dependencies:
- dependency-name: actions/setup-node
  dependency-version: '6'
  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@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 20
       - uses: pnpm/action-setup@v4

.github/workflows/npmpublish.yml

@@ -24,7 +24,7 @@ jobs:
       -
         uses: actions/checkout@v5
       -
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 20
       - uses: pnpm/action-setup@v4
@@ -74,7 +74,7 @@ jobs:
         with:
           fetch-depth: 0
       -
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 20
           registry-url: https://registry.npmjs.org/