Use the 'main' branch for checkout

Author: Duncan Ferguson

.github/workflows/dzil_tester.yml

@@ -36,7 +36,7 @@ jobs:
     name: Perl ${{ matrix.perl-version }}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@main
       - name: Regular tests
         if: ${{ !matrix.coverage }}
         run: |