Single-thread interaction with Artifactory

There is a regression in repo.spring.io that causes artifacts from
the same module to have SNAPSHOT replaced with different timestamps.
This then breaks dependency resolution in consuming builds. It's a
long-shot, but JFrog have been unable to identify that cause of the
problem so we're going to try using a single thread for uploading
to Artifactory to see if that helps.

ci/pipeline.yml

@@ -248,7 +248,7 @@ jobs:
       build_uri: "https://ci.spring.io/teams/${BUILD_TEAM_NAME}/pipelines/${BUILD_PIPELINE_NAME}/jobs/${BUILD_JOB_NAME}/builds/${BUILD_NAME}"
       build_number: "${BUILD_PIPELINE_NAME}-${BUILD_JOB_NAME}-${BUILD_NAME}"
       disable_checksum_uploads: true
-      threads: 8
+      threads: 1
       artifact_set:
       - include:
         - "/**/spring-boot-docs-*.zip"