Update Checkout action default branch

GitHub Actions gives a warning that the checkout action no longer has
'master' as the default branch. This is because the checkout actions
repo itself has renamed its default branch from 'master' to 'main'.

The action may eventually break when the 'master' branch option is
no longer supported.

Let's update the CI workflow to use actions/checkout@main instead of
actions/checkout@master, to fix the warning.

.github/workflows/gradle.yml

@@ -11,10 +11,10 @@ jobs:
 
     steps:
       - name: Set up repository
-        uses: actions/checkout@master
+        uses: actions/checkout@main
 
       - name: Set up repository
-        uses: actions/checkout@master
+        uses: actions/checkout@main
         with:
           ref: master