Bump actions/checkout from 2 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
         rust: [1.70.0, nightly]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Install rust
         uses: actions-rs/toolchain@v1
         with:
@@ -57,7 +57,7 @@ jobs:
         rust: [nightly]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Install rust
         uses: actions-rs/toolchain@v1
         with:
@@ -84,7 +84,7 @@ jobs:
    name: Rustfmt
    runs-on: ubuntu-latest
    steps:
-     - uses: actions/checkout@v2
+     - uses: actions/checkout@v4
      - uses: actions-rs/toolchain@v1
        with:
          profile: minimal
@@ -101,7 +101,7 @@ jobs:
     name: Clippy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
@@ -123,7 +123,7 @@ jobs:
     name: Docs
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
@@ -150,7 +150,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.ref == 'refs/heads/master'
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Build docs
         env:
           GITHUB_REPO: ${{ github.repository }}