Upgrade actions/checkout to v3

.github/workflows/ci.yml

@@ -19,29 +19,29 @@ jobs:
   markdownfmt:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - run: docker pull tianon/markdownfmt
       - run: .ci/check-markdownfmt.sh
   ymlfmt:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - run: docker pull tianon/ymlfmt
       - run: .ci/check-ymlfmt.sh
   short:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - run: .ci/check-short.sh
   required-files:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - run: .ci/check-required-files.sh
   no-readme:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
         with:
           fetch-depth: 0
       - run: .ci/check-pr-no-readme.sh