ci: upgrade checkout action to v4

This update is necessary as GitHub Actions will stop supporting
Node.js 16. Version 4 now uses Node.js 20.

Reference:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

.github/workflows/ci.yaml

@@ -10,7 +10,7 @@ jobs:
  name: Code Formatting Check
  runs-on: ubuntu-latest
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
  - uses: dtolnay/rust-toolchain@nightly
  with:
  components: rustfmt
@@ -34,7 +34,7 @@ jobs:
  pull-requests: write
  runs-on: ubuntu-latest
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
  - uses: dtolnay/rust-toolchain@nightly
  - name: Generate test result and coverage report
  run: |
@@ -58,7 +58,7 @@ jobs:
  runs-on: ubuntu-latest
  if: github.ref == 'refs/heads/master'
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
  - uses: docker/login-action@v2
  with:
  registry: ghcr.io