Add script verifying libbeat dependency can be upgraded

[simitt]

In the past we had CI scripts running checking that libbeat can be updated to the latest according version (master, 7.x). The script being removed in #3371 seems unintentional. The usage was commented out in the Jenkinsfile, suggesting the script not being used anymore.
However, it is still run from https://beats-ci.elastic.co/job/elastic+apm-server+7.x+beats-update/ and https://beats-ci.elastic.co/job/elastic+apm-server+master+beats-update/

The test runs still have the same value than before the cleanup and switching to go modules.

[axw]

Oops, sorry. I intentionally removed it thinking it was unused.

[graphaelli]

I think elastic/beats#17218 resolves this issue.