(deps): Bump actions/checkout from 6 to 7

Bumps [actions/checkout](https://github.com/actions/checkout) from 6 to 7.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v6...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-lean.yml

@@ -38,7 +38,7 @@ jobs:
 
       - name: A1) Checkout repository code
         # WHY: Needed to access files for checks.
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
 
       - name: A2) Install uv
         # WHY: The E2) se-validate-manifest pre-commit hook shells out to `uvx`,
@@ -66,5 +66,5 @@ jobs:
       # BUILD: Get code and build Lean project
       # ============================================================
 
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
       - uses: leanprover/lean-action@v1

.github/workflows/links.yml

@@ -37,7 +37,7 @@ jobs:
 
     steps:
       - name: 1) Checkout repository code
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
         # WHY: Required so Lychee can inspect repository files.
 
       - name: 2) Run Lychee