Bump actions/checkout from 3 to 4 (#562)

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/main.yml

@@ -21,7 +21,7 @@ jobs:
         # Running mypy-protobuf itself
         py-ver-mypy-protobuf: [3.8.17, 3.9.17, 3.10.12, 3.11.4]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Read version numbers
         run: |
           echo ::set-output name=PROTOBUF_VERSION::$(grep "^protobuf>=" test_requirements.txt | cut -f2 -d=)
@@ -88,7 +88,7 @@ jobs:
     name: Linting
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: actions/setup-python@v4
       - name: Run formatters and linters
         run: |
@@ -106,7 +106,7 @@ jobs:
     name: Sanity Check Windows Executable
     runs-on: windows-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: actions/setup-python@v4
       - name: Read versions
         run: echo ::set-output name=PROTOBUF_VERSION::$(grep "^protobuf>=" test_requirements.txt | cut -f2 -d=)