Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Install Rust stable
         uses: dtolnay/rust-toolchain@stable
         with:
@@ -39,7 +39,7 @@ jobs:
       checks: write
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Install Rust stable
         uses: dtolnay/rust-toolchain@stable
         with:
@@ -55,7 +55,7 @@ jobs:
     name: doc
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install Rust nightly
         uses: dtolnay/rust-toolchain@nightly
       - name: Run cargo doc
@@ -76,7 +76,7 @@ jobs:
       #   if: runner.os == 'Windows'
       # - run: vcpkg install openssl:x64-windows-static-md
       #   if: runner.os == 'Windows'
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install Rust
         uses: dtolnay/rust-toolchain@stable
       # enable this ci template to run regardless of whether the lockfile is checked in or not
@@ -92,7 +92,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: smver-checks
         uses: obi1kenobi/cargo-semver-checks-action@v2
         with: