[skip ci] disable cron on 0.8.1 (#2711)

* [skip ci] update pymilvus-test version
Signed-off-by: Nzw <zw@milvus.io>

* [skip ci] update cron on 0.8.1
Signed-off-by: Nzw <zw@milvus.io>

* [skip ci] remove ubuntu ci
Signed-off-by: Nzw <zw@milvus.io>

* [skip ci] update cron
Signed-off-by: Nzw <zw@milvus.io>

* [skip ci] disable cron on 0.8.1
Signed-off-by: Nzw <zw@milvus.io>
Co-authored-by: Nzw <zw@milvus.io>

ci/jenkins/Jenkinsfile

 #!/usr/bin/env groovy
 
 String cron_timezone = "TZ=Asia/Shanghai"
-String cron_string = BRANCH_NAME == "0.8.1" ? "H 11 * * * " : ""
+String cron_string = BRANCH_NAME == "master" ? "H 11 * * * " : ""
 
 pipeline {
     agent none