Bump actions/checkout from 3 to 4

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] <[email protected]>

.github/workflows/test.yml

@@ -37,15 +37,15 @@ jobs:
           - neovim: false
             version: v8.2.0000
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Vim
         uses: rhysd/action-setup-vim@v1
         id: vim
         with:
           neovim: ${{ matrix.neovim }}
           version: ${{ matrix.version }}
       - name: Setup vim-themis
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           repository: thinca/vim-themis
           path: vim-themis