Only run snyk for 7.17 and main branch (#103820) (#103844)

Author: breskeby

.buildkite/pipelines/periodic.yml

@@ -1281,7 +1281,7 @@ steps:
       image: family/elasticsearch-ubuntu-2004
       machineType: n2-standard-8
       buildDirectory: /dev/shm/bk
-    if: build.branch == "main" || build.branch =~ /^[0-9]+\.[0-9]+\$/
+    if: build.branch == "main" || build.branch == "7.17"
   - label: Check branch consistency
     command: .ci/scripts/run-gradle.sh branchConsistency
     timeout_in_minutes: 15