Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/mix_test.yml

@@ -14,7 +14,7 @@ jobs:
         otp: [25.1, 24.3.4]
         elixir: [1.13.4, 1.14.0]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac # v4
 
       - uses: erlef/setup-beam@v1
         with: