chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -19,9 +19,9 @@ jobs:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
       - name: checkout
-        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5.0.1
+        uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
       - name: checkout vim-themis
-        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5.0.1
+        uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
         with:
           repository: thinca/vim-themis
           path: vim-themis