Wait for version-qualifier.sh script to land upstream

Use  elastic#17311 once it lands and gets
backported to 8.x. For now just hard code version.

Author: donoghuc

.buildkite/pull_request_pipeline.yml

@@ -137,8 +137,8 @@ steps:
     command: |
       set -euo pipefail
       source .buildkite/scripts/common/vm-agent.sh
-      # Provides QUALIFIED_VERSION
-      source ci/version-qualifier.sh
+      # TODO: Use https://github.com/elastic/logstash/pull/17311 to compute QUALIFIED_VERSION once merged
+      QUALIFIED_VERSION="8.19.0-SNAPSHOT"
       ./gradlew --stacktrace artifactDockerObservabilitySRE
       docker run docker.elastic.co/logstash/logstash-observability-sre:${QUALIFIED_VERSION} \
         logstash -e 'input { generator { count => 3 } } output { stdout { codec => rubydebug } }'