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](https://github.com/actions/checkout/compare/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>

.github/workflows/build.yaml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
       - name: Install Go
         uses: actions/setup-go@v4
         with:

.github/workflows/docs.yaml

@@ -9,10 +9,10 @@ jobs:
     steps:
       -
         name: Checkout ntfy code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
       -
         name: Checkout docs pages code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
         with:
           repository: binwiederhier/ntfy-docs.github.io
           path: build/ntfy-docs.github.io

.github/workflows/release.yaml

@@ -25,7 +25,7 @@ jobs:
       NTFY_TEST_S3_URL: ${{ secrets.NTFY_TEST_S3_URL }}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
       - name: Install Go
         uses: actions/setup-go@v4
         with:

.github/workflows/test.yaml

@@ -25,7 +25,7 @@ jobs:
       NTFY_TEST_S3_URL: ${{ secrets.NTFY_TEST_S3_URL }}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
       - name: Install Go
         uses: actions/setup-go@v4
         with: