Update compatibility-phoenix.yml

Author: rrrene

.github/workflows/compatibility-phoenix.yml

@@ -26,7 +26,7 @@ jobs:
           - otp: 22.3
             elixir: 1.14.0
         repo_url: ["https://github.com/phoenixframework/phoenix.git"]
-        repo_branch: ["v1.7", "main"]
+        repo_branch: ["v1.6", "main"]
     steps:
       - uses: actions/checkout@v2.3.1
       - uses: erlef/setup-beam@v1