chore(deps): update actions/checkout action to v3 (#142)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/checkout](https://togithub.com/actions/checkout) | action |
major | `v2` -> `v3` |

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.github/workflows/copybara.ci.yml

@@ -20,7 +20,7 @@ jobs:
       (github.event_name == 'pull_request' && contains(github.event.pull_request.labels.*.name, 'migrate:protocol'))
     steps:
       - name: "Setup: Checkout"
-        uses: actions/checkout@v2
+        uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3
       - name: "Migrate: Protocol"
         uses: sgammon/copybara-action@5669453d54072738035b0985cfe322d0ee5de0db
         with:
@@ -48,7 +48,7 @@ jobs:
     steps:
       # Pull code.
       - name: "Setup: Checkout"
-        uses: actions/checkout@v2
+        uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3
       - name: "Migrate: Cluster"
         uses: sgammon/copybara-action@5669453d54072738035b0985cfe322d0ee5de0db
         with: