build(deps): bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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] <support@github.com>

Author: dependabot[bot]

.github/workflows/audit.yml

@@ -11,7 +11,7 @@ jobs:
   audit:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Setup
         run: cargo generate-lockfile
       - name: Audit

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
         os: [ubuntu-latest, windows-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Install dependencies (windows)
         if: matrix.os == 'windows-latest'
         run: |
@@ -33,7 +33,7 @@ jobs:
   lints:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Clippy
         run: cargo clippy --all-targets -- -D warnings
       - name: Fmt
@@ -42,21 +42,21 @@ jobs:
   pre-commit:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
       - uses: pre-commit/action@v3.0.1
 
   doc:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Doc
         run: cargo doc --all-features
 
   check-external-types:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
           toolchain: nightly-2025-08-06
@@ -72,7 +72,7 @@ jobs:
   unused-deps:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
           toolchain: nightly

.github/workflows/msrv.yml

@@ -11,7 +11,7 @@ jobs:
   msrv:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Setup
         run: cargo install cargo-msrv --locked
       - name: Msrv WTransport-Proto