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>

Author: dependabot[bot]

.github/workflows/diff-lock.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checkout_repo
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: install_rust
         uses: dtolnay/rust-toolchain@stable
       - name: generate_lockfile

.github/workflows/fmt.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checkout_repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: install_rust
         uses: dtolnay/rust-toolchain@stable

.github/workflows/linter.yml

@@ -14,7 +14,7 @@ jobs:
     continue-on-error: true
     steps:
       - name: checkout_repo
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: install_rust
         uses: dtolnay/rust-toolchain@stable

.github/workflows/msrv.yml

@@ -16,7 +16,7 @@ jobs:
     continue-on-error: true
     steps:
       - name: checkout_repo
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: install_rust
         uses: actions-rs/toolchain@v1
         with:
@@ -48,7 +48,7 @@ jobs:
         ]
     steps:
       - name: checkout_repo
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: install_rust
         uses: dtolnay/rust-toolchain@stable
       - name: install_cargo_msrv

.github/workflows/test.yml

@@ -35,7 +35,7 @@ jobs:
             rust: stable
     steps:
       - name: checkout_repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       # We would prefer to use `dtolnay/rust-toolchain@master with toolchain=${{ matrix.rust }}` or `rustup update ${{ matrix.rust }}`.
       # However, when using either we run into a linking issue: