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/beamer.yaml

@@ -11,7 +11,7 @@ jobs:
   beamer:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: cagix/pandoc-lecture-zen@master

.github/workflows/docsify.yaml

@@ -11,7 +11,7 @@ jobs:
   docsify:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: cagix/pandoc-lecture-zen@master

.github/workflows/gfm.yaml

@@ -11,7 +11,7 @@ jobs:
   gfm:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: cagix/pandoc-lecture-zen@master

.github/workflows/pdf.yaml

@@ -11,7 +11,7 @@ jobs:
   pdf:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: cagix/pandoc-lecture-zen@master

.github/workflows/tests.yaml

@@ -11,7 +11,7 @@ jobs:
   test_beamer:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: cagix/pandoc-lecture-zen@master
@@ -25,7 +25,7 @@ jobs:
   test_gfm:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: cagix/pandoc-lecture-zen@master
@@ -39,7 +39,7 @@ jobs:
   test_docsify:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: cagix/pandoc-lecture-zen@master
@@ -53,7 +53,7 @@ jobs:
   test_pdf:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: cagix/pandoc-lecture-zen@master