Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 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/v4...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/backend-ci.yml

@@ -24,7 +24,7 @@ jobs:
         working-directory: backend
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
@@ -63,7 +63,7 @@ jobs:
         working-directory: backend
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable

.github/workflows/docker.yml

@@ -22,7 +22,7 @@ jobs:
       packages: write
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -63,7 +63,7 @@ jobs:
       packages: write
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3

.github/workflows/frontend-ci.yml

@@ -21,7 +21,7 @@ jobs:
         working-directory: frontend
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Node.js
         uses: actions/setup-node@v4