Revert "ci: Use Zephyr topic-sdk15 branch for testing"

This reverts commit 5899236.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ on:
       zephyr-ref:
         description: 'Zephyr Ref (branch, tag, SHA, ...)'
         required: true
-        default: topic-sdk15
+        default: main
       host:
         description: 'Host'
         type: choice