Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/latexmk.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     runs-on: ubuntu-20.04
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: yegor256/latexmk-action@0.8.1
         with:
           opts: -pdf