Revert "Switched to actions/checkout@v4 to avoid CI deprecation warni…

…ngs."

This reverts commit a8750d5.

checkout@v4 doesn't work on ubuntu:16.04 and ubuntu:18.04 images:

actions/checkout#1590

.github/workflows/ci.yml

@@ -313,7 +313,7 @@ jobs:
             fi
             git config --global pack.threads 0
 
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v3
 
       - name: Install packages
         if: matrix.install
@@ -533,7 +533,7 @@ jobs:
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v3
 
       - name: Setup Boost
         shell: cmd