Bump actions/checkout from 3 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...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/jig.yml

@@ -16,7 +16,7 @@ jobs:
     env:
       GRADLE_OPTS: '-Dorg.gradle.daemon=false'
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
       # 画像出力のためにGraphvizを入れます。
       # 自分で apt install してもいいです。DOT（テキスト）でいい場合は不要です。