[Chore] Update "actions/checkout"

Problem: node16 is now deprecated and github-runner provided by nixpkgs
no longer supports this runtime. However, "actions/checkout@v3" uses
this runtime.

Solution: Update CI pipeline to use "actions/checkout@v4".

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
  name: Verify cross references
  runs-on: ubuntu-latest
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
  - uses: serokell/xrefcheck-action@v1
  with:
  xrefcheck-version: 0.2.2
@@ -60,7 +60,7 @@ jobs:
  ghc: 8.10.7
 
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
  if: github.event.action == 'opened' || github.event.action == 'synchronize' || github.event.ref == 'refs/heads/master'
 
  - uses: haskell/actions/setup@v2
@@ -99,7 +99,7 @@ jobs:
  ghc: ["9.4.5"]
 
  steps:
- - uses: actions/checkout@v3
+ - uses: actions/checkout@v4
  if: github.event.action == 'opened' || github.event.action == 'synchronize' || github.event.ref == 'refs/heads/master'
 
  - uses: haskell/actions/setup@v2