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] <support@github.com>

Author: dependabot[bot]

.github/workflows/default.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out the repo
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Docker meta
         id: meta

.github/workflows/nightly.yml

@@ -12,7 +12,7 @@ jobs:
         java: [17]
     steps:
       - name: Checkout GitBucket repo
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: gitbucket/gitbucket
           path: gitbucket
@@ -37,7 +37,7 @@ jobs:
           sbt executable
 
       - name: Check out the repo
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           path: docker