[CI] Only trigger DRA workflow on intake for main branches (elastic#114077)

Author: brianseeders

.buildkite/pipelines/intake.yml

@@ -76,6 +76,7 @@ steps:
   - trigger: elasticsearch-dra-workflow
     label: Trigger DRA snapshot workflow
     async: true
+    branches: "main 8.* 7.17"
     build:
       branch: "$BUILDKITE_BRANCH"
       commit: "$BUILDKITE_COMMIT"