chore(): fix GitHub workflow branch names

.github/workflows/main.yml

@@ -4,9 +4,9 @@ name: CI
 # events but only for the master branch
 on:
   push:
-    branches: [ master ]
+    branches: [ main ]
   pull_request:
-    branches: [ master ]
+    branches: [ main ]
 
 env:
   DESECSTACK_DOMAIN: your.hostname.example.com