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/dart.yml

@@ -23,7 +23,7 @@ jobs:
         with:
           path: "~/.pub-cache/hosted"
           key: "os:ubuntu-latest;pub-cache-hosted;dart:stable"
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: dart-lang/setup-dart@v1
       - name: "Dependencies"
         run: dart pub upgrade --no-precompile
@@ -47,7 +47,7 @@ jobs:
     name: "Flutter CI"
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: amake/dart-flutter-compat@v1
         with:
           channel: stable