ci: 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](https://github.com/actions/checkout/compare/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] <support@github.com>

.github/workflows/backport.yml

@@ -31,7 +31,7 @@ jobs:
         with:
           app-id: ${{ vars.CI_APP_ID }}
           private-key: ${{ secrets.CI_APP_PRIVATE_KEY }}
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           token: ${{ steps.app-token.outputs.token || secrets.GITHUB_TOKEN }}
           ref: ${{ github.event.pull_request.head.sha }}

.github/workflows/github_pages.yml

@@ -12,7 +12,7 @@ jobs:
         os: [ubuntu-latest]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable

.github/workflows/tag-maintainers.yml

@@ -23,7 +23,7 @@ jobs:
           app-id: ${{ vars.CI_APP_ID }}
           private-key: ${{ secrets.CI_APP_PRIVATE_KEY }}
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: ${{ github.event.pull_request.base.sha }}
       - name: Get Nixpkgs revision from flake.lock

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
       hm: ${{ steps.changes.outputs.hm }}
       tests: ${{ steps.changes.outputs.tests }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: dorny/paths-filter@v3
         id: changes
         with:
@@ -47,7 +47,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Get Nixpkgs revision from flake.lock
         id: get-nixpkgs
         run: |

.github/workflows/update-flake.yml

@@ -34,7 +34,7 @@ jobs:
             echo "email=$id+$name@users.noreply.github.com"
           } >> "$GITHUB_OUTPUT"
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: ${{ matrix.branch }}
           token: ${{ steps.app-token.outputs.token || secrets.GITHUB_TOKEN }}

.github/workflows/update-maintainers.yml

@@ -48,7 +48,7 @@ jobs:
             echo "email=$id+$name@users.noreply.github.com"
           } >> "$GITHUB_OUTPUT"
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           token: ${{ steps.app-token.outputs.token || github.token }}
       - name: Get Nixpkgs revision from flake.lock

.github/workflows/validate-maintainers.yml

@@ -15,7 +15,7 @@ jobs:
     if: github.repository_owner == 'nix-community'
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Get Nixpkgs revision from flake.lock
         id: get-nixpkgs
         run: |