ci: disable scorecard check on non-`master` branches

https://github.com/emqx/emqx/actions/runs/7960101073/job/21728381092#step:4:269

```
2024/02/19 13:11:45 creating scorecard entrypoint: validating options: only default branch is supported
refs/heads/release-55 not supported with push event.
Only the default branch master is supported.
```

.github/workflows/scorecard.yaml

@@ -10,7 +10,6 @@ on:
   push:
     branches:
       - master
-      - 'release-5[0-9]'
   workflow_dispatch:
 
 permissions: read-all