ci: bump actions/checkout from 4 to 7

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 7.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v7)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '7'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     name: fmt · clippy · build · unit + fixture tests
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
 
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
@@ -82,7 +82,7 @@ jobs:
           - { os: macos-latest,  asset: haybarn_unittest-osx-arm64.zip }
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
 
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
@@ -119,7 +119,7 @@ jobs:
     name: metadata quality (vgi-lint)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
 
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
@@ -142,7 +142,7 @@ jobs:
     name: supply-chain audit (cargo-audit)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
       - name: cargo audit
         uses: rustsec/audit-check@v2
         with:
@@ -153,7 +153,7 @@ jobs:
     name: MSRV check (rust 1.90)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
       - name: Install Rust toolchain (pinned MSRV)
         uses: dtolnay/rust-toolchain@1.90
       - uses: Swatinem/rust-cache@v2