Change default branch for CI builds to 'main'

Signed-off-by: Ryan Baxter <ryan.baxter@broadcom.com>

.github/workflows/ci.yaml

@@ -5,7 +5,6 @@ on:
     branches:
       - main
       - 4.3.x
-      - 4.2.x
 
   # Scheduled builds run daily at midnight UTC
   schedule:
@@ -17,7 +16,7 @@ on:
       branches:
         description: "Which branch should be built (can be a comma-separated list of branches)"
         required: true
-        default: '4.3.x'
+        default: 'main'
         type: string
 
 jobs: