Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  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

@@ -16,13 +16,13 @@ jobs:
     steps:
       -
         name: Install etherpad core
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: ether/etherpad-lite
           path: ./etherpad
       -
         name: Checkout plugin repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           path: ./plugin
       -

.github/workflows/npmpublish.yml

@@ -15,14 +15,14 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       -
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: ether/etherpad-lite
           path: etherpad-lite
       -
         run: mv etherpad-lite ..
       -
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       -
         uses: actions/setup-node@v4
         with:
@@ -70,7 +70,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       -
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
       -